In the menu from your username in the top-right, click "Custom filters" (alternately, go to
http://project-gc.com/User/CustomFilters )
Click New
Give it a name and click Add
Select the Attributes tab, find the attribute you want, select it and click Add as required
Click Add to filter, then Save
Go to Map Compare. From the Add filter dropdown, select Custom filter.
In the Custom filter dropdown, select the filter you just created. Add other filters as appropriate and click Filter.