ok, so the command prompt
 
 In Windows, it's in Start -> Programs -> Accessories -> Command Prompt. Or you can do Start -> Run -> type in 'cmd'
 
 In the command prompt, you want to change directory to the directory where bspc is, so for example if gtkradiant is in c:\gtkradiant\
 
 cd \gtkradiant
 
 then type what I said in the above answer.
 
 bspc -bsp2aas c:\quake3\baseq3\maps\mymap?.bsp
 
 this runs the program, and lets you also start it with command line options (like '-bsp2aas')
 
 If you need more help with using the Command Prompt, go 
here