Under Mac when I try to run "geth", I get the error "command not found".
Mist is installed and in use..
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[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?