I can agree the signs for the lounge are virtually non-existing, there is just a sign at the bottom of the stairs, but if you don't know where the stairs are then it's a bit harder.
Perhaps it is a deliberate move to make sure they don't get random people to the lounge who are not supposed to be there?!

I'm a bit surprised the CDG flight arrived at the F-pier, I have flown ARN-CDG plenty of times and these flights almost always arrive at the A-pier, the F-pier if almost exclusively used by non-Schengen flights (TLL for example).
If you arrive at the A-pier it is usually a lot easier to find your way around, signage is a bit better.

As already mentioned Arlanda, just like many other airports, has grown a lot in the past 10 years which has forced the authorities to expand the airport.
From the beginning there was only one international terminal with 20 gates, the one which is now called terminal 5. Back then finding your way around was very easy.
Then terminal 2 opened and was partly used as an international terminal, and eventually all remaining domestic flights were moved from that terminal to make space for more international traffic.
And now just 3 (?) years ago the F-pier opened.
These are just a few reasons why the airport is somewhat large, confusing and illogical.

Why don't you write a short letter to Luftfartsverket, the Swedish Airport Authorities, and tell them about your experience?!
It seems like they are struggling to make Arlanda as good and passenger-friendly as possible, so I'm sure they wouldn't mind getting some feedback.