AdvancedMapViewer.apk file will install the Advanced Map Viewer App on your Device. (Note you may have to go to SETTING --> SECURITY and CHECK the UNKNOWN SOURCES option in order to do the install on your Android Device).
Then go to ftp://download.mapsforge.org/ maps/north-america/us/ and download any maps that you want to keep on your Android Device. (They are listed by state and run about 60mb each on the average and have a .MAP extension). Once again you can download these .MAP files straight to your Android device or to your computer then transfer them to your Android Device. You should put the .MAP files in the FOLDER (MAPS) you created earlier.
1. To set C:GEO to use the offline maps you do not need the ADVANCED MAP VIEWER APP just the .MAP files for the states you want to keep on your phone.
2. Start the C:GEO App. Use your MENU Button to go to SETTINGS. Scroll down in the settings menu and find the MAP SOURCES section (probably set to GOOGLE MAPS).
3. There should be a DIRECTORY WITH OFFLINE MAPS section. CLICK on the "..." and pick the Folder on your phone or tablet where you put the .MAP files. Your MAP SOURCES should now be "OSM: Offline-MAPNAME".