The "Resources" folder contains all the images, sounds, and config files that define the levels, towers, enemies, and even the player. They're organized by type to help make things easier to find. For instance, all of the images are in the "Images" folder while the audio is in the "Sounds" folder. You're likely most interested in the "Templates" folder at first since it contains the config files for the enemies, towers, and projectiles. All of the images in the "Images" folder are bitmaps and all the sound files in the "Sound" folder should be wave files.
Before I start describing each file format, it's worth mentioning the resources.list file. This file lists the resources that you want the game to load and the order in which they're loaded. You declare sections (Images, Sounds, Sprites, etc.) that define what type of file you're loading and then give a list of files along with the ID you want to use to reference that resource in other files. For instance, to load an image, you might say
Images
tower = my_tower.bmp
Then in other files you could reference that image by just saying "tower". Each ID must be unique within it's type, but other types can reuse it. So you couldn't have two images with the ID "fireball", but you could have an image and a sound both named "fireball". This description isn't super precise, but it's a pretty simple file and you should be able to get the structure just by looking at the one in the "Resources" folder.
Now, on to the list of custom file types. They're all plain text, not binary, so you can edit them in Notepad or your favorite text editor.