Under Mac when I try to run "geth", I get the error "command not found".
Mist is installed and in use..
No. Mist includes geth.
The reason you're getting the
command not found error is because geth is not recognized by your command line path. Running Mist will automatically start the local copy of geth.
[An addendum to the main answer concerning the Mac implementation of Geth.]
Not all implementations of Mist explicitly include Geth.
If you're using the pre-built packages on the Releases page, the 32-bit versions for Linux and Windows don't include Geth, whereas the 64-bit versions do. (As does the Mac version.) In the case of the 32-bit versions, Geth should be installed separately.
See this previous thread: How do I install Mist from the pre-built binaries on 32-bit Linux?