Dear Marco, you can find DSSP at http://www.embl-heidelberg.de/dssp/ or could prefer use Procheck from the CCP4 series (http://www.ccp4.ac.uk/) and run it on your pdb file by the "procheck [pdbfile] [chain number] [resolution]" command. You will obtain 10 postscript files in which you can find drawn in the 6th one the secondary predicted structures that you can write in your helix and sheets part in your pdb.
Hoping that it could help you See ya Virgile ADAM
