Can't advise as to the correct port but there are members who advise against doing this as home repair.

At the very least, I'd advise using a new O ring, otherwise you may well create a leak point.

A search should reveal all.
Always start your entry with 'SLKWorld.com' to get forum specific results.

For instance:

SLKWorld.com R170 a/c refill gives:
Mercedes Benz SLK Forum - Search Results for SLKWorld.com R170 a/c refill

I'll leave you to filter through that lot.

 

An A/C system that needs to be recharged is an A/C system that has a leak. It is designed and only works properly as a closed system - only needing refrigerant after repairs are made and all leaks resolved. Refrigerant does not just go away or get used up. We are all here to help each other and my best advice is to have a fully trained tech undertake the needed repairs and recharge the system with the proper volume by weight of refrigerant. If you still wish to undertake this on your own, it does look like you have identified the proper low pressure port, but without having the car in front of me I can not say for sure. Best of luck.

 

An A/C system that needs to be recharged is an A/C system that has a leak. It is designed and only works properly as a closed system - only needing refrigerant after repairs are made and all leaks resolved. Refrigerant does not just go away or get used up...

The performance of the system has declined VERY slowly over the course of 2 or 3 years. That makes me relatively confident that the cause is a slow leak. I don't anticipate keeping the car for more than another year or so. If I can solve the problem by applying a $20 solution to the problem, even if I have to do it once a year, that seems like a bargain to me. Realistically, the solution only needs to last about 4 months until the need for A/C goes away in the fall.

 

port location solved

With thanks to Robrowdy, and his confirmation, I connected to the low pressure port and topped of my refrigerant. For future readers, it is the port shown in the first photo next to the cylindrical component with the green sticker. My pressure was low but nowhere near gone, so there can't be much of a leak involved.

It made a big difference when the car was moving, but A/C was still warm when the car was stopped. This prompted my to check and sure enough the radiator fan is not working. Before I start taking things apart, is there an easy way to check to see if the fan needs to be replaced or if the problem is the switch that controls the fan?

 

There is a connector to the fan located under the coolant overflow tank. You can access it by removing the tank, or disconnecting the large black tubing held together with worm gear bands next to it. Disconnect the fan connector and connect test leads from the battery, if you have some that are long enough. If not, you will need a spare 12v battery or 12v power source, or remove your battery from the car to get it close enough for your lead to reach.

With the connector disconnected, you can also get a reading with a multimeter from the fan control box. It is the rectangular silver box on the fender well.