Documentation 
Section: (Usage) 3.5.11. To Use Functions
3. Click on the Value field, then enter the new value. Use the keyboard shortcuts to invoke a Calculator button. For example, enter 90K to calculate sine(90).

Correct version:
A couple of simple working examples. A Farenheit to Celcius conversion example might be nice, if possible. Can the function use the number already on the display or is it not really a function so it can only hold a constant?

Other information:
Even though I'm not using the most recent version, I didn't see a bug report in bugzilla and I couldn't find the change in the changelog so I'm posting this bug. The help file says you should be able to create a function, that takes the sin of 90 degrees, by entering 90K in the value field of the Edit Functions dialog box. It doesn't seem to work. Only simple expressions such as 2+3*4=  gave results. It took a lot of trial and error to figure out how to get it to give me anything but a malformed function error. While the function syntax seems obvious in hindsight, I kept trying various things like standard spreadsheet notation =2+3*4 and plain old 2+3*4  Is there any way to include the display value so that the user functions are actually functions and not just constant expressions? I'm running Ubuntu 6.06