> copy the PGN file from one directory into the aquarium directory

What you want to do is not very clear.

Why don't you just open the pgn file in Aquarium?

Start Aquarium, click on "database" in the left bottom corner to go to the database pane, then click on "Database" at the top of the ribbon, then click on "Open" on the left of the ribbon, select the appropriate pgn file and open it.

Otherwise, open the pgn file in any text editor like Notepad, select the game, copy it with Ctrl+C and then paste into the "Notation" window of Aquarium (Sandbox pane or IdeA pane) with Ctrl+V.

So, basically you're asking how to copy a file in Windows, right?
Windows 7: http://pcsupport.about.com/od/windows7/ht/copy-file-windows-7.htm
Windows XP: http://pcsupport.about.com/od/windowsxp/ht/copy-file--windows-xp.htm

Just so you know, database files can be anywhere on your computer, they don't have to be in Aquarium directory. But it's fine if you want to keep the files in one place
When you mentioned Aquarium directory, I sure hope you meant AquariumData\ABases (or Aquarium\ABases if you haven't created separate data folder under the installation)

And when you want to open your PGN database, you just switch to Database tab on the ribbon and click on Open.

Thanks - worked like a charm!