You need to use the attached reg editor.
1. Copy the regeditSTG2.exe to the root directory of your storage card using a DIRECT card reader/writer FROM YOUR PC. You CANNOT copy this file to your phone using the explore option from activesync. If you do not have a microSD reader/writer for your PC, you're out of luck, sorry.
2. Put the microSD back in your phone, and open regeditSTG2 from file manager.
3. Change the following values as per previous WM5 installations
HKEY_LOCAL_MACHINE\Security\Policies\Policies\00001001 = 1
HKEY_LOCAL_MACHINE\Security\Policies\Policies\00001005 = 40
That's it, you're done. I can now run unsigned apps on my phone.