We are using Natural Selection 1, version 3.2. There is not, and won't ever be again, another update for this mod. But the client.dll Natural Selection uses is from Half-Life 1, and is still subject to updates.
I recently noticed that there was a steam platform update for HL1 so I just updated the servers today.
If you are still unable to join our servers, then try the following.
In your games list, find Half-Life, right click on it and select "properties".
Click the LOCAL FILES tab at the top.
Select "Verify Integrity Of Game Cashe.."