How do I get my Bitcoin Core to rescan for a different wallet.dat file on OSX?
The QT terminal (RPC) rescan option will be available in 0.16 (not release by the time of writing). https://github.com/bitcoin/bitcoin/pull/7061

Manual rescans should generally not be required, though, if you are missing transactions in an old wallet, the rescan may help.

In Core &lt; 0.16 you need to rescan either by startup argument or by a configuration option.

Startup (OSX)

Open up "Terminal.app" and enter /Applications/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt --rescan

bitcoin.conf (OSX)

Place a plain text file called bitcoin.conf in ~/Library/Application Support/Bitcoin/ and make sure the textfile contains the line "rescan=1".

Make sure you remove the rescan=1 line after you have successfully rescanned your wallet.dat.