
Open a new Finder window and navigate to /System/Library/Extensions. Make two backup copies of AppleUSBMultitouch.kext (one as a safety net, and one to edit), and then perform the modifications below to the editing copy.
First, enter the .kext file (using Show Package Contents), then go to Contents, then open Info.plist (I usually open it in Property List Editor that comes with Xcode Tools (which come on the OS X Install discs, but other text editors should work too). If it opens in Property List Editor, then do the following:
- Go to IOKitPersonalities and open it
- On all the sections begining with WSTrackpad*3 (like WSTrackpadISO3) scroll down until you get 'TrackpadThreeFingerDrag.' Copy that line (it should have a tick next to it).
- Paste it in all the sections begining with WSTrackpad that don't already have it.
- Save the changes.
Note: You MUST repair permissions before the reboot, otherwise you'll lose keyboard and trackpad functionality completely. If this happens anyway, you can try booting from another disk and copying the safety net backup to the original location, then repair permissions, and reboot again.
This only works on MacBookPro4,1 or later and MacBookAir1,1 or later. Older models will keep the same nonmultitouch trackpad preference pane. I have only tested this on a MacBookPro4,1. Newer models might not have a difference in the preference pane. Credit for this hint goes to this Macrumors forum thread. There are some pictures in that thread of how the edited version should look.
[crarko adds: I haven't tested this one. Be absolutely certain to make the backups, understand the copy procedures, and get the permissions repair done, or you'll likely be doing a system reinstall. It would be wise to have a current Time Machine backup and the correct OS X Install discs handy too, just in case. I don't have one of the mentioned machines to try this out on; if any of you do try this please tell us all how it went.]