Hi,
G52 is a sub datum, to use this you must specify a position from your datum to the sub datum point e.g.
G52 x2. y4. z5.
Once you have done this you can then use g41/42 to offset your cutter.
G21 G40 G00 G90
G52 X2. Y4.( shift datum )
G00 X0 Y0 (move to new zero)
Z.1
G01 Z-.5 F2.
G41 X10. F10. (apply comp)
G03 I-10. (mill circle)
G40 X0 (cancel comp)
G00 Z1.
G52 X0 Y0 (cancel shift)
M30
Graham.