I haven't tried it myself, but the wiki on the github lists several sources for pdb files:
The third link is at MyGet: MyGet PDBs
Since you're pulling the packages from MyGet, you should probably look there for PDBs too. ServiceStack 3.x is in maintenance mode and won't be upgraded (except for bugfixes) so for now it seems the new versions are only posted to MyGet and not at all to NuGet.