Hi,
simple way is to mount the tape and then issue:
scanner -iv -S(savesetid) <drivename>
this will then scan the tape and rebuild the client index entries (and media if necessary) for the saveset you want. It will scan the entire tape so could take some time.
Alternatively, to scan just the saveset and fast-forward to the right part of the tape do the following:
mminfo -r'volume,mediafile,mediarec' -qssid=(savesetid)
scanner -ivf(mediafile) -r(mediarec) -S(savesetid) <drivename>
Hope this helps