Have you looked in the help file for how to enable Rumba support, this is what I found ...
Enabling and Disabling Rumba
Rumba is the world's premier Windows desktop terminal emulation solution. Rumba provides connectivity solutions to mainframes, mid-range, UNIX, Linux, and HP servers.
Before you can record and replay Rumba scripts, you need to enable support:
- Install Rumba desktop client software version 8.1 or later.
- Click Start > Programs > Silk > Silk Test > Administration > Rumba plugin > Enable Silk Test Rumba plugin.
- Click Start > Programs > Silk > Silk Test > Administration > Rumba plugin > Disable Silk Test Rumba plugin.