Why doesn't 3 MSN Messenger work on the KU990?

[Isobutane]

I've recently got a 3 PAYG sim, and one of the main reasons was for the free use of MSN Messenger. However, my phone was originally on O2, so thus doesn't have 3 firmware, so it took me ages to find the right settings for 3 to use the internet. Anyway, I finally found them, and the download for MSN went fine, but I can't sign in. Every time I load up the "WL Messenger" application, I get the error message "This application requires the network usage. Please start again under network coverage".

I obviously have signal when I try, anyone know what could be the cause of this? Is it something to do with the fact the phone doesn't have 3 firmware?