ARC Button

I placed the three point arc button in my tool bar.
When I attempt to use it, I get an UNKOWN COMMAND ERROR.
Suggestions??
We experienced this before with some of the standard tool bar buttons also.
What are we doing wrong?
When I attempt to use it, I get an UNKOWN COMMAND ERROR.
Suggestions??
We experienced this before with some of the standard tool bar buttons also.
What are we doing wrong?