How recharging is done depends on your carrier, as there are different recharging methods.
Most likely you'll have to enter a top up code by a so-called USSD code (something like *123#). Allthough Mobile Partner generally supports USSD, the corresponding function is hidden and unhiding it is a bit complicated.
Therefore I would suggest you
download a tool called MDMA. Afterwards fully close Mobile Partner (through
File | Exit) and start MDMA - in the lower left you will see an input field, where you can enter any USSD code. When you've entered the USSD code click "Send USSD".