It is not in Trisquel's repository because it is not upstream (in Ubuntu). As
long as you trust whoever administrated the PPA (apparently, the developers
of Retroshare), you can follow the instructions on the page you found.
- [Trisquel-users] Retroshare not in repo? freeworld4
- [Trisquel-users] Re : Retroshare not in repo? lcerf
- Re: [Trisquel-users] Retroshare not in repo? santiago . lorente