For your GO600, you do not use internet tethering (not WiFi, and not Bluetooth tethering -- it's done in a special way that does not require this feature on your phone). You need an application that works on Android and iPhone that is available from TomTom to make the connection between the two. Please let us know if you have the Android application loaded on your phone yet.
The 1st thing, though ... for the application to work, your phone must support the Bluetooth SPP profile. I am not all that familiar with the CAT range of phones, but the information I have shows that the B15 phone as supplied by Rogers supports only a very slim list: A2DP, AVRCP, OPP/FTP, HFP, and HSP profiles. If that is true, then you will not be able to pair the GO unit to your phone.