If you are on Rogers and you have Firmware 2.1 you shouldn't have to add in any APN settings, because there already there by default.
Make sure you don't have edge disabled in BossPrefs.
If you had the phone unlocked my someone other then yourself it's possible that they removed the APN string from your iPhone.
To edit the APN you will need to Install "APN Editing" located in Cydia.
Once that's installed you can find it in Settings > General > Network > Cellular Data Network.
You can refer to the following page if you like.
iPhone: How to view or edit the APN setting