To perform an MP2 calculation of the NMR shieldings you have to prepare the input with mp2prep -c.
mpshift will then calculate both the SCF and MP2 shielding constants. The result is written into the control file after the keyword $nmr mp2 shielding constants.
The script mp2prep will create the keywords
and starts a statistics run of mpshift (by calling mpshift). If the resulting disk space requirement exceeds the automatically detected free disk space on your system, it will increase $traloop and run a statistics run again. This will be done as long as your free disk space is not sufficient for the calculation.
If the mp2prep script fails to run on your system, try to use the -p option or do the procedure described above by hand. Call mp2prep -h for more informations about mp2prep.