There's a few ways to do this in Mastercam. (That's a typical response for Mastercam related questions, because there's usually several ways of doing things)
The easiest way is to open the operation and click T/C Plane button on page one of the operation dialog. To the left of the next dialog, check the box "Work offset", and enter a value for the offset you want.
-1 = next available, (usually G54) This is the default value.
0 = G54
1 = G55
2 = G56 etc.
Another way is to create a WCS with an offset assigned to it. You can create any number of WCS, and assign different offsets to each. This is a more involved and dificult way, but is much better for bigger programs. (I often have up to and in excess of 150 operations per part, with up to 6 different offsets/WCS's, and the latter way is the only way to go on programs like that.)
HTH
Matt
San Diego, Ca
___ o o o_
[l_,[_____],
l---L - □lllllll□-
( )_) ( )_)--)_)
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)