There are multiple options for displaying the MS2 fragment masses both theoretical and observed. To view the theoretical masses click the table icon:
This will open the following table which displays the theoretical masses along with options to show the observed and delta mass differences between the two. Highlighted in red are the masses that were found in the spectrum, black those that were not observed.
To label the mass values click the cog wheel:
Then select Show x values:
This will label the mass values for all signals in the MS2 plot: