For Catznip its under Me>Preferences>Catznip, check "RestrainedLove API Support" and restart.
For BlackDragon its under Dragon>Edit>Preference>Restrained Love, check "Enable Restrained Love (A)" and restart
Most other Third Party viewers will be similar to Catznip.
The official Linden Lab viewer last time I looked did not support the RLV API.