The best place to look in "extras" repository for your OS version. The trick to installing is to have the correct repositories referenced in the Applications Manager. Place the following 3 lines in your 770 in a text file called repository.install and execute that file. Maemo Mapper should then be available to you for the 770.

[install]
repo_name = Maemo Garage (new gregale)
repo_deb = deb http://repository.maemo.org/extras/ gregale free non-free