I used to use the Blackberry Web Client (BWC). I have now had my admin at work connect me to the office blackberry enterprise server (BES).

My BWC gets email from a pop account I have elsewhere (wyne1234@mypopaccount.com)
I can connect to the office Lotus Notes email (wyne1234@notesaccount.com).

When I send email from my blackberry I want to be able to choose which email address will show as the return address.

My admin will not support this here at work however they did say that they have seen people that have it set up so that when they send mail they can choose which account it will use.

I cannot find how to do this anywhere, any help would be much appreciated.

you say you have both BES and BWC (now BIS) - therefore you'll have a [cmime] service book for each account and therefore can choose whatever address you want

just change "send using" when you compose an email