You can locate the folders that the different extensions belong in by selecting Tools → User Content and then selecting the folder for the type of item you want to place within ArtRage. In many cases, you will then need to close and reopen the ArtRage program for the files to be loaded into the program. In the cases where you have .arpack files, simply double click the file in your computer folder and it will automatically open ArtRage and open a prompt where you can select which items you'd like to install from the package.
Nothing is easy to the unwilling.