you're gonna have to be more descriptive in what you're trying to do. are you trying to get RR variables read in flash or flash variables read in RR? The point of SETFLASHVAR is to have variables you set in RR to display in flash. For instance. SETFLASHVAR;TRACKCOUNTVAR;1000 would display the number "1000" in a dynamic text you create in flash named _root.TRACKCOUNTVAR. Basically its a way to display RR variables just like we display RR labels.
If you wanted to set a flash var in RR you would set the variables content in flash then use SETVAR to get it into RR. For example
fscommand("SETVAR;SPRITECOUNTVAR;" add _root.spritecount)