Hi Marc,
What steps are you taking to try to select something?
A typical set of steps to select a single note might be:
1. Open a .mid file in Notation Composer.
2. Click the "Select" mode button in the toolbar.
3. Click on the 2nd quarter note in the first measure of the top staff.
Giving me some precise steps for your method of selection will help me to narrow down the possibilities.
It looks like from your screen shot that you're pressing the "S" key and clicking in a staff. The "S + mouseclick" shortcut is used for Staff, not Select. (I also see that it's not included in the Help/Users Guide - Appendix A - Keyboard shortcuts either - gotta fix that.)
If you want to use a keyboard shortcut to help in selecting a region, Ctrl + mouse drag is the one you'd want.
If the above "notes" (pun intended

) don't help, please do let me know a bit more detail.
ttfn,
Sherry