How do you type a true minus sign (not a hyphen) on a Mac?
There's a way of entering Unicode values on the Mac: go to System Preferences, then Language &amp; Text. In the Input Sources tab, scroll to Unicode Hex Input, check it.



Then if you switch to Unicode Hex Input in the Mac menu bar, you can press
&#x2325;+2212 for the minus sign. Here's what the input switcher looks like:



If you want to switch between your input and hex input via keyboard, you have to click "Keyboard Shortcuts..." in the same window, disable "Show Spotlight Window" in the Spotlight section, and maybe activate "Select next source in Input Menu". &#x2325;+&#x2318;+space will then swap input language. (You can check this in the menu bar.)

(Alternatively, there's the low-mouse-use option via Special Characters:
Go to Edit > Special Characters...;  in there, select the Unicode tab, and Code tables (or however it's called in English) at the top.
Next, scroll down the list until you see 00002200 in the Unicode column. Click it. Down below, look for the row that begins with 2210, and it should be the 3rd char.)