If you are looking for the sight glass (the window you mentioned), there may not be one. Not all A/C systems have them. If there were one it would likely be on top of your receiver-drier, but looking at your picture there isn't one there - only the high-side service port. Your low-side lines will be larger in diameter than your high-side lines. What you need to do to find the low-side service port is follow the larger line coming off the compressor. If you follow it all the way back to the firewall, you're bound to find the low-side fitting. Though the low-side line is larger than the high-side line, the low-side fitting will be smaller, and it may have a platic cap with an "L" on it. Like Scottie said, the fitting may be on the compressor itsself. It could be anywhere between the compressor and the firewall.