Hi Kerry,
Can you give us your definition for this modification? Its m/z, of course, will depend on the charge state. e.g. Label:13C(6)15N(2) (C-term K) is 8 Da, but 4 m/z on a doubly charged precursor.

You should see the neutral monoisotopic and average calculated masses in the Edit Structural Modification form where you are defining your modification. (see attached PNG file) If you could post a similar screenshot of your modification definition, that would help a lot.

--Brendan