Default should work fine. If you don't know what BES is (BlackBerry Enterprise Server/Service), you probably don't have it!
Have you tried using the app? Is it failing? If it does, try using Direct and set your APN as follows:
Go to ( Settings/Tools --> ) Options --> Advanced Options --> TCP and enter the following settings:
APN: wap.voicestream.comAlternate APN: internet2.voicestream.com or internet3.voicestream.com with [blank] username and password.
Username for APN: [blank] or Your T-MOBILE Username
Password for APN: [blank] or Your T-MOBILE Password
The APN settings come from this page: http://www.pinstack.com/carrier_sett...n_gateway.html