Download the new skin/scenery/etc. It will probably be zipped and will contain various .pcx/.bmp files as well as a readme.txt and .gif or .jpg preview.
The readme should tell you where to put it.
Aircraft skins usually go in FA-18/Ncape/Objects while terrain and sky pallette files usually go in FA-18/Ncape
Unzip the files to some temporary directory.
From ART MANAGER, click on the category (SKIN ART, SCENERY, etc.) and click ADD
Give the NAME of the addon, the author, and choose what category it belongs in from the drop down box. IF a .gif or .jpg file was included for preview purposes, click browse and browse to it in the temporary location you unzipped the files to. Now click NEXT
Again, browse to the temporary file and highlight all the files necessary for the skin/terrain. CLICK ADD to add them to ART MANAGER.
Click FINISH
NOW..from the correct category, click on the new entry, and click EDIT. Make sure the path (NCAPE or NCAPE/OBJECTS) is correct. If not, then correct it and hit SAVE, otherwise you may CANCEL.
Now select and click INSTALL TO F/A-18 and your new files will be added.
It is always a good practice to find the files of the same name and back them up. Using the procedure above, you can add them to ART MANAGER as 'ORIGINAL XXX' where XXX describes them. This way you can easily replace them with Art Manager.