Hi,
We are still workin' on the issue you mentioned, however there is an option for you: go to the main page, do a simple flight search, check Priceline, Travelocity and others then search for 'add a car rental' option on their site.
Don't hesitate to ask about any further problems or questions! :) You can also register to the forum and travel community of travelgrove, their travel experts will answer all of your questions! Here you can find it:
http://www.travelgrove.com/community/
Regards and have a nice trip! - (2009-05-14)