I'm guessing $SMPROGRAMS is a global variable in NSIS.

One typically defines variables like so:

!define PLACEINSTARTMENU "$SMPROGRAMS\${MYOTHERVARIABLE}"

 
 

This suggests that $SMPROGRAMS will just be available however. I guess there is a difference between a dollar sign followed by letters and a dollar sign followed by letters in curly brackets. Oh, boy... Installer! :P

 
 

Addendum: "environmental variable" is probably a better way to describe $SMPROGRAMS in lieu of: "global variable"