Ok, well I'll just assume you want to go back to CM when you're done, and you can modify these steps or pick a different route accordingly.
1) Right now you have Clockwork 3.0.0.5. Without going into too much detail, I'll just say that you won't be able to flash certain things with that. For now, you should use Clockwork 2.6.0.1. If you open ROM manager, you should have the option to flash it. If not, select the option to flash an alternate recovery. It should be Amon RA 2.2.1 or 2.3 (not sure what version ROM manager is up to now). Since you don't have any nandroid backups yet or any real reason not to switch, go ahead and just leave it at that. RA is a good recovery.
2) In ROM manager, select the option to reboot into recovery. Once you do, the phone will reboot and boot back up into recovery. It will be whatever version of RA that you flashed. Select the option that says nandroid, then backup. It will prompt you for what to backup. Choose boot, system, data, cache (not all that necessary), and wimax. Let it backup. It will take a few minutes.
3) Go
here and download the zip file that is linked to in that thread. It is a rooted version of the 3.29.651.5 OTA, with no other changes. Let me know if you want to actually stick with a Sense ROM, and we can discuss a better ROM. For now, though, if all you want to do is get it activated, then go back to CM, this will do fine.
4) Copy the zip file to the root (top level) of your sdcard. Reboot into recovery, again, then select the option to wipe. Wipe data, cache, and dalvik cache. Once it is done, go back, then select the option to flash a zip from the sdcard. Once you are given a list, select the zip file that you downloaded. Follow the prompts to flash it.
5) Once it is done, reboot, and the phone should boot up to a clean Sense ROM. At that point, call Sprint, and have them activate it. Do *not* accept any OTA updates if prompted, and do not do it manually if instructed by Sprint. Your phone is still rooted, and you *will* mess up root. You have the eng bootloader, so you'd likely lose root altogether (assuming unrevoked was never used).
6) At this point, if you'd like to go back to CM, reboot into recovery, select the nandroid option, then select restore. You will be given a list of previous backups. Select the backup you made at the beginning. Once it is done, you should be back in CM, with an activated phone. Note that you only have to activate the phone once. Switching ROMs does not undo it.
ALTERNATE METHOD:
Since your radios are a little out of date, you might consider updating them to the latest version. You *might* have an issue restoring your nandroid backup, but you'd probably be fine. You have the latest stable CM, so that's good. If you want to do this, do everything through step 2 (finish your backup), but instead of flashing that one ROM, flash the PC36IMG.zip version of
this ROM. As you're just going to a generic Sense ROM temporarily, this is the easiest method. Follow the instructions on that page to flash it. It's done from hboot, not recovery. Once it is done flashing, you can boot up, and have the phone activated. After that, you should be able to restore your nandroid backup. You will have a different radio version, though. If it causes a problem, you can reflash CM or downgrade your radio. We can help you with either if it's an issue.
I will definitely check at home, where I have a charger
