PPTP Connection tracking will be supported by default in v5.26 firmware. However in the current firmware it's possible to add some lines to the expert config:
http://support.deliberant.com/forums/p/932/4289.aspx#4289
(see 5 extra lines)
To add these, you'll have to either load the AP 2i firmware, which will give you access to expert mode, or SSH in to the device, and use vi to edit /tmp/system.cfg. After adding these lines and saving, you'll need to run sysconf -w to apply temporary config to flash.
Hope that helps, let us know if you have any questions.
-Matt