Both of the above replies are correct. And yes, it does depend on your interface. If you can post the exact models of your PT interface, and your Mackie mixer, then we can take a look. Is there a particular reason you want to use the Mackie with Pro Tools? Are you using it for tracking? Mixing?
A little background...
This is a topic that is pretty technical and can get a little complicated at first.
-18=0 is an incomplete statement. A more complete statement would be -18dBFS = 0VU = +4dBu.
dBFS is a digital signal level that only exists only in the digital domain and doesn't have an absolute correlation to any analog voltage. dBu is an analog voltage level referenced to 0.775 volts RMS (AC). So 0dBu = 0.775 Vrms, and +4dBu = 1.228Vrms.
The D/A converters are calibrated to match a specific digital level (anywhere from -14dBFS to -20dBFS) to a specific analog level (usually +4dBu, or 1.228Vrms, for line level balanced outputs). 0VU is a further calibration to match a specific analog level (+4dBu) to a reading on a VU meter (0VU).