Yeah you'll first want to meta-doctor your Pre Plus to webOS 2.1.
Then, when the phone boots for the first time, it will go to the activation screen (and probably say Activation Failed or something similar, makes no difference).
Once there, type "##" on the keyboard, which will bring up the phone dialer. Continue dialing 3386633# using the touchscreen, so that you have ##3386633# in the dialer (aka ##DEVMODE#). This will bring up the dev mode enabler. (Note: if your Pre Plus is a AT&T/GSM model, then the first "##" to be typed is a "#*" instead)
It will show that dev mode is already on, so you actually have to turn it OFF, then reboot, then go back and do ##338633# and turn dev mode ON again. The phone will reboot once again, and this time you will arrive at a normal home screen.
Once you are at this point, install Preware, and then use Preware to install Impostah, which you can use to create a new palm profile for that phone, or log into an existing one. After that you're all set...full functionality with a legitimate profile, but without phone activating phone service.
Btw, once you're up and running, you can also put the phone into airplane mode, but then turn Wifi and/or Bluetooth back on individually, so that the 3G radio stays turned off (since you won't need it) to conserve battery without affecting anything else. The phone will remember this arrangement after a reboot too.