Bear in mind that Audacity does not officially support Yosemite yet, and you didn’t tell us your version of Audacity (see the pink panel at the top of the page).
If you don’t have at least Audacity 2.0.6, get it here: Audacity ® | Download for Mac OS. Be sure to follow Installation: Do NOT run Audacity from the DMG!!!.
Open Finder, Go > Go to Folder and type:
~/Library/Application Support/audacity/
Open audacity.cfg, delete all the text then type in:
NewPrefsInitialized=1
If audacity.cfg does not exist, create it containing just
NewPrefsInitialized=1
at the top.
It it still won’t work, you may have permissions problems in /var/folders/.
You can force audacity.cfg to use any location for its temp folder, for example:
[Directories]
TempDir=/Users/<your user name/audacity_temp/
That will work (Audacity will create the folder if it doesn’t exist) as long as you have permission to write in or create folders there.
Gale