The ruler on the tool bar, next to the knife, becomes active when you open a glyph window. Also Alt + 4 turns it on. On my PC control + alt + shift changes the arrow to the measuring tool while you hold it down. The only problem with that is that it will sometimes switch the keyboard to another language, as alt + shift changes the keyboard to another language on a PC. As you are on a Mac it's probably a bit different.

I think the question was about the measurement line, not the ruler. The button in the metrics panel is the one you should be using. However, I´ve found the measurement line to be a very buggy feature, usually disappearing completely in the font after a while...

Oh, my I misunderstood, sorry. I think FontLab is more stable on the PC, so I've never had that problem :)

usually disappearing completely in the font after a while...

That must be what happened. Annoying.
I rarely use it. But it does come in handy occasionally in italics.

By the way, you can also check/uncheck it in Preferences/Glyph window/Appearance

Nick,

The measurement line sometimes goes far away to the space on both platforms.
To make it visible again use the Python script below. Open your font, open the Edit Macro panel, put this small program there and click on the Run Macro button.

fl.font.measurement = 400
fl.UpdateFont()

These two strings will set the Measurement line for the current font to 400 units.

Regards,
Johnych

Thanks!

For non-Latin scripts, the Measurement line is really handy - I use it quite frequently. Last night it suddenly disappeared from the font and I couldn't find a way to make it visible again. Thanks to this thread and Johnych for the solution.