How to trace a net in Altium Designer viewer (search for all places a signal is used)?
I think what you need is the "Navigator-Panel" of Altium. 
On the bottom right click on "Design Compiler" then on "Navigator".  Now "compile" your project.


Altium provides a useful manual:

http://techdocs.altium.com/display/ADRR/WorkspaceManager_Pnl-Navigator%28%28Navigator%29%29_AD

There is also a good video from Robert Feranec on the topic of how to propperly check your schematic. It can be found on Youtube:

https://youtu.be/CDAuCchchtI