Go to
http://www.e4me.ae/e4me/etisalat/portal and register.
Then click "Mobile Services"
https://www.e4me.ae/e4me/etisalat/login at the top right-ish side.
Then choose "Recharge existing Wasel Account".
Click "View & Continue".
And then everything else is straight forward.
You will also need to have the mobile phone under your own name to do that.
If it all doesn't work, go to any of the Etisalat, Co-op or HSBC outlets (most other banks too) where there is a re-charge machine and you can pre-pay any amount you want (min. AED50).