How do I adjust the frequency for an external...

Some people have had problems hooking up Mac laptops to projectors at other institutions (where, often, the support people know nothing about Macs). It is sometimes suggested to try reducing the frequency of the monitor output.
If you just use the control strip widget, only the "Recommended" settings for an external monitor will be shown. You need to open the Monitors control panel while the external monitor or projector is attached. Then, above the list of pixel resolution and frequency settings, choose "All". You will then be able to select the "suboptimal" settings that might get the projector to work (e.g., 1024 by 768, 60 Hz instead of 75 Hz).
Also see the caveat above about using the LitePro -- some projector will accept higher pixel resolutions than their native mode, which then tend to look crappy.