This works for me with my cheapo t-Zones account, there may be better settings for the regular data plan. This does get 3G though (heh).
Go to settings...wireless controls...mobile networks...access points. Create a new AP:
Name: tmo (or whatever)
Username, Password, Server, MMSC: (not set)
MMS Proxy: 216.155.165.050
MMS port: 8080
APN type: internet+mms
Authentication Type: None
what do you mean they want you to update your plan? If you have data allowance on your current plan, wtf, why cannot they ust give you the settings!
Then I guess you just don't have prepaid internet access in your current plan... What do you mean you can see 3G network? 3G is not internet, it is indeed network, over which you can access the Internet, or can not if you're not paying for that.
If you could provide a link to your plan details on T-Mobile's web-site it would be easier do advice you.