See under 'Download' for an rsync-based mechanism to obtain all PDB files, and for the FTP address of the executables. Feel free to FTP (actually better use the rsync mechanism) all DSSP files from our FTP site. Occasionally, we make small corrections in some files, so your mirror script should check the time stamps/file sizes. If you have problems FTPing the files (something like "argument list too long" or "too many files"), then you are strongly urged to see the rsync stuff.
Feel free to use the software and the data. You are not supposed to redistribute the software beyond your own group. You are not supposed to distribute DSSP files beyond your own institute.
You can use this web server form to get a DSSP file for your in-house PDB file. Please do not use this server for the calculation of the secondary structure of existing PDB files (those can be obtained either by FTP, or by rsync, or using MRS). We've created a web server to calculate DSSP files from PDB files. You can find the server at http://www.cmbi.ru.nl/xssp/.
In case of problems, feel free to contact me via e-mail at "Gerrit(dot)Vriend(at)radboudumc.nl". I am not the author of DSSP, but I will help if I can.