if I understand you correctly, you want the plus and minus buttons on 
your Apple Remote mapped to system volume increase and decrease when 
SlingPlayer is the frontmost application.

That's easy to achieve:

1) Go to Preferences > Mapping > SlingPlayer 
2) Select the Increase Volume and Decrease Volume actions from the 
"Audio, CD & DVD" Behaviour for the respective entries of the Plus 
and Minus buttons in the table (including the "hold" versions if you 
want).

Those are the only changes needed in a stock install. Remote Buddy 
will then even display a huge, translucent volume bar.

Hope that helps.

Best regards, 
Felix