[inquisitor]

Originally Posted by mgol

Anyway, I have entered in my account and tried (with the help of Google translate) to find out my status. I think I understand what happend. I have activated data option XL and since it is not supported by the free prepaid plan, I got zero (well 0.01 Euro) credit.

Actually you never had more than € 0.01, since this is the initial amount with which the SIM is sold. If you have tried to activate the "Internet Flatrate XL" with this insufficient amount of credit it simply failed.
Since you posted at the PWIA-wiki, that you did not provide any bank account details, you are definitely not on a postpaid plan.
So everything seems to be okay except for the free "Internet Flatrate L" not being activated for you. But this can only be solved by complaining with customer care that you never received that free month.

1. How can I make sure my above assumption is correct and I have not done another mistake (e.g. opening the comfort account, etc.) ?

From my findings you haven't mistaked anything. The SIM is obviously active and it's on a prepaid plan. The mistake seems to be on Tchibo's part.

2. Can I somehow top up my credit while in Israel (e.g. by using credit card) or the only way is to buy a voucher (Tchibo or O2) while landing in Germany ? Transferring money to Tchibo's bank account looks somewhat too complicated compared to the above.

Yes, you can by topping up with vouchers from O2 Germany (those are sometimes called "O2 Loop Up"), which are sold online by several companies. Unfortunately most of them like So macht man das! - Aufladen.de don't accept foreign credit cards.
I know of Prepaid-Karte & Handy aufladen – Prepaid online aufladen bei Cabox.de to accept international customers paying with PayPal, after activating a code, that was previously sent by snail mail.
Also there are O2 top up vouchers available at eBay.de, but you won't be able to buy one if you're registered outside Germany and the seller doesn't offer shipping outside Germany. In that case contact the seller and ask him to enable international shipping, as this will allow you purchase the voucher and the seller can then send you the top up code by eMail.
If you finally get the top up voucher just enter it in MDMA as described above.

3. Should I activate the data option again or am I sticked with the same option (I believe XL) for the rest of my life ?

Generally these data options are recurring, so once activated if there's enough credit left at the end of those 30 days, Tchibo will automatically deduct the amount for the next 30 days. But if you run out of credit the tariff option will be stopped. Then you have to top up your credit and activate the data option again.
The problem is we don't know if Tchibo have enabled the Internet Flatrate L for you in the meantime. If that's the case, you run the risk to get the Internet Flatrate L for another month if you top up credit now and you can't change data options before the end of that 30 day period.
To find out if a data option is activated start MDMA and send *104#, which will show you the

Optionsmanager

1 Optionsstatus
2 Option buchen

Now just send "1" (without * or #) to check the status of your tariff options:

Folgende Optionen sind gebucht:

1 Internet-Flatrate L
0 Abbruch

If you actually get an option listed (as in the example above) you can send "1" again and you should get the details for this option:

Ihre Option Internet-Flatrate L ist aktiv und läuft am 31.08.2010 aus.

If the Internet Flatrate L is active, you should disable it or wait with your recharge until the option has expired. I don't know if you can cancel tariff options through the USSD-menu, but you can definitely do it by sending an SMS message containing "STOP DATENFLAT2" to 7777 or just wait the option to expire.

If your subscription of the Internet Flatrate L has finished you can top up at least € 20 of credit and order the Internet Flatrate XL by by sending *104# with MDMA.

Optionsmanager

1 Optionsstatus
2 Option buchen

Now send 2 to subscribe to an option.

Folgende Optionen sind für Sie buchbar

1 Festnetz-Flatrate
2 Tchibofon-Flatrate
3 100 Minuten-Option
4 50 Minuten-Option
5 weitere
0 Abbruch

Send 5 to see the following page of options.

Folgende Optionen sind für Sie buchbar

1 50 SMS-Option
2 Internet-Flatrate L
3 Internet-Paket 500 MB
4 Internet-Tages-Flatrate
5 weitere
0 Abbruch

Again send 5 to see the next page.

Folgende Optionen sind für Sie buchbar

1 Internet-Paket
2 Internet-Flatrate XL
0 Abbruch

Now send 2 for the Internet-Flatrate XL.

Die Option kostet 19,95 Euro monatlich.
Möchten Sie die Option Internet-Flatrate XL buchen?

1 Ja
0 Abbruch

Finally you need to confirm your subscription by sending 1.