The Open With... action uses the system's default for opening the
application. If you cannot see it in the list, then are you sure OS X knows
to use TaskPaper to open the file? Right click the file and open the 'Open
With' menu. Is TaskPaper there?
If not, try either making TaskPaper the default app for .taskpaper files or
reloading launch services. Run this in Terminal:
On 10.8 Mountain
-kill -r -domain local -domain system -domain user
Well, as of B71, it should list all applications in the catalog, with the default app (according to the OS) listed first.
But something's up. If I recall, the code removes the default app from the list and re-inserts it at the top. I think the last step is failing. The third pane is always blank for Open With… and the default app for the selected file is nowhere to be found.