You must add these lines to your .cshrc file. This example is for SGI, but it is not difficult to modify it for other platforms.
In any case, do not forget to alter the paths (displayed in red) to point at the proper location on your system.

#----------- CHECK WHERE WE HAVE LOGGED IN -------------
#      and set default path for AMBER and CHARMm
#-------------------------------------------------------
 
switch (`uname -a | awk '{ print substr($0,1,4) }'`)
 
   case "IRIX":
	setenv CHARMHOME /disk11/local/ProServer/Charmm
	setenv AMHOME /usr/bioinf/bin/OM/AMBER
	setenv amber /usr/bioinf/bin/OM/AMBER/amber41
	setenv pamber /usr/bioinf/bin/OM/AMBER/amber41/parallel_exe
	switch (`hinv | grep 'CPU:' | awk '{print $3}'`)
	
		case "R4400"
			set path = ($amber/mips2_exe $path)
			echo "CPU: R4400"
			breaksw
	
		case "R10000"
			set path = ($amber/mips4_exe $path)
			echo "CPU: R10000"
			# --- check if we can run the parallel version
			set nbproc = `hinv | grep MHZ`
			if (`echo $nbproc | awk '{print $3}'` == "MHZ") then
				set nb = `echo $nbproc | awk '{print $1}'`
				if ($nb > 1) then
					echo $nb processors
					alias psander '$pamber/sander'
					alias pgibbs '$pamber/gibbs'
				endif
			endif
			breaksw
 
		default:
			echo "CPU: unknown (by script!)"
			breaksw
 
   default:
	breaksw
 
endsw