After you buy the SIM card, and register it, get the recharge slip that you want. Put the SIM card in a phone temporarily. Now, dial the number listed on the included slip of paper to set your language to English. Do your top up, dial 125, selet the data package you want.
Look at the chart at the bottom of:
3.5G Data Packages
You can buy a monthly bucket of data on any Wasel prepaid (you might have to buy the regular Wasel rather than the traveler, I'm not sure on that). You just buy a regular Etisilat SIM card and register it. You then buy the appropriate prepaid top up slips and but them on the phone. Then dial 125 and go through the recordings to add the package you want.
The thing that I suspect is confusing you is the price of 99AED for 10gigs which is cheaper than even the 1 gig offering and only 5AED more expensive than 100 meg offering. I can't explain it either. I emailed a friend about this and will supplement my answer.
Since I got an unlimited global data package from ATT, I haven't shopped prepaid data market as much as I used to. I think the catch is on the top of the page which says it is a three month only offer and seems to suggest that you have to pop down the 300 AED at once to get it.