(I'm assuming you already downloaded Ancient Remains)
Step 1: Use punctuation.
Step 2: Go to the folder where you installed Tremulous, which is probably C:\Program Files\Tremulous
Step 3: Unzip the Ancient Remains file. (Unless, of course, you already did)
Step 4: Copy or move the .pk3 file you got to the "base" folder inside your Tremulous folder. No, not the base2 folder.
Step 5: Learn all about the wonders of the forum search tool.
Step 6: Enjoy!