Just copy the archive to the directory where the other packages live . For example, ttgap/pkg/ in your home directory. Or use the tt pkg/ directory in one of the paths from the GAPvariable tt GAP_ROOT.
Then call GAPand type
gap> LoadPackage("rds"); ---------------------------------------------------------------- Loading RDS 0.9beta5 by Marc Roeder For help, type: ?RDS ---------------------------------------------------------------- true
For a test, see the example in chapter RDS:A quick start.
RDS manual