On a Mac WordPress via Xampp you cannot install themes or plugins because of the settings. In a terminal do like this:
# sudo -i
# (write your password)
# cd /Applications/Xampp/xamppfiles
Now find out where your WordPress files are and cd to the relevant folder, then do something like this:
# chmod -R 777 wordpress/
# chown daemon:daemon wordpress/
Now ... this is a quick and very, very dirty fix. In a production environment the folders should have chmod 755 and the files chmod 644.