Merkaartor is developed using Git at http://gitorious.org/merkaartor. You can use that to compile the bleeding-edge development version for yourself, or to contribute your improvements to the code.
Bug reporting is managed on this site, via the Issues tab, above. After logging in you can create new ones in the New issue tab.
The mailing list is http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/merkaartor