I am going to have to assume that you may not have configured the input. From the manual:

To choose which input is routed through the Rig:
1 Hold the Edit/Back button for one second to bring up the User Options screen.
2 Use the scroll wheel to select the Rig Input User Option.
3 Press the SW1 button to enter the Rig Input menu.
4 Set the input you wish to route through the Rig with the illuminated control knob.
5 Press Edit/Back twice to return to your original view.

To ensure you hear your audio when using Eleven Rack as a standalone signal processor, be sure to set the Rig Input User Option to the type of input you plan to use (Guitar,
Mic, Line).

If that does not work check the line level switch, it may need to be in the +4db mode

If you do not have it, highly recommend downloading the manual (http://akmedia.digidesign.com/support/docs/Eleven_Rack_User_Guide_v90x_71670.pdf) lots of good stuff in there