The seond life viewer has "File, Edit, View, World, Tools and Help." along the top.
If you can't see Advanced, press Ctrl, Alt and D together - that will bring up the Advanced option.
Click on Advanced. Click 'Character'. From the next sub-menu, click on 'Character Tests'. Finally, click on 'Go Away/AFK When Idle'. Make sure this doesn;t have an X next to it.
Second Life