If you have a certain amount of WMZ in your WM purse, open keeper on your PC and keep it open.
Then, go to the billing and select "deposit" under the word "balance" (in the left upper corner of the page).
You will be redirected to Fund account page.
Select payment method (in your case - Webmoney). Enter the amount of money required (don't forget about % charged by WM for transfer). Click Pay by Webmoney button.
You will be redirected to WM Transfer page. Select the type of keeper used (Classic or other installed on your PC). Fill in all fields required. That's all. After transfer is complete, check your balance and continue purchasing package(s) you need. That's all.
After all payment operations are completed, close your WM keeper.