OSX Lion 10.7 appears to come with an improper installation of Python 2.7. Because of this, REAPER will crash immediately when opening the Actions window. (Or if REAPER was saved with the Actions window open, it will crash immediately on startup.)
We will release REAPER version 3.77 soon that disallows Python 2.7 on OSX and falls back to 2.6 (which is also shipped with OSX Lion), and REAPER 4.0beta16 will have the fix as well. We will also add a user preference to prevent REAPER from even looking for Python/Perl.
In the meantime, the workaround is inconvenient but will prevent the crash.
1. Open the file /Users/<username>/Library/Application Support/REAPER/reaper.ini. Clicking the file should open it in TextEdit.
Annoying note! OSX Lion makes it much harder to access <username>/Library. By default, that folder will not appear in the Finder. The "official" way is to open the Finder, open the Go menu, then hold down the Option key. Alternatively, open Applications/Utilities/Terminal and type "chflags nohidden ~/Library". This Lion behavior will cause all sorts of inconvenience for advanced REAPER users who are familiar with the REAPER inifiles.
2. In the reaper.ini file, locate the line that contains just the word "[REAPER]". Immediately below that line, insert the line "pythonlib=libpython.2.6.dylib".
When you restart REAPER, it should not crash.