A timely thread. I just encountered the same problem and the same solution worked for me. I am using 64 bit Win7. I just did a fresh install of MapSource, then tried to install Colorado topo 2011 but it was not showing up. After trying various uninstall/re-install/restart combos, I came across this thread. Installing My Trails did the trick. @jbensman, thanks. @ineedmaps1 thanks for bringing the problem to the forum.
Bud
Bud