I do have the same problem.
Im using the latest version of Qlandkarte M, of february 8th 2010.
I tried it this way:
On your computer, in qlandkarte GT, export the geotiff file.
A file with the same name, and the extension qmap is formed.
The map will be visible in the database of QLandkarte GT on your computer.
.
Copy both files from your computer to the MyMaps directory on the N900.
Open qlandkarte M on the N900.
Choose 'menu', and browse to the 'map'menu.
Hit the button "rebuild map database".

Now the map should become visible in the database list.
In my case, however, nothing happened.