ok, I don't know what I was doing wrong but this is what ended up working:

Code:
function ErrorLog($ErrorMsg)
	DIM	$LogFile,$LogError,$AltLogFile,$error,$serror,$ErrorString

	$error			= @ERROR
	$serror			= @SERROR
	$ErrorString		= "Error $error,  $serror, $ErrorMsg, @WKSTA, @USERID, @DATE, @TIME, @CRLF"
	$LogFile			= "\\server\SoftwarePushes\sms\logging\@MDayNo_@MonthNo_@Year.log"
	$AltLogFile			= "\\server\SoftwarePushes\sms\logging\@MDayNo_@MonthNo_@Year-@WKSTA.log"
	$LogError			= OPEN (5,$LogFile,5)

	If $LogError=0
		$ = WRITELINE(5,$ErrorString)
	Else
		$ = OPEN (6,$AltLogFile,5)
		$ = WRITELINE(6,$ErrorString)
		$ = CLOSE (6)
	EndIf
	$ = CLOSE(5)
endfunction