I am having a lot of trouble getting two of my mods to work. I downloaded "Deadly Clutter" and "Redcourt Castle" mods and It show the yellow "!" boxes for all of deadly clutter, and half of redcourt castle. I don't have a "Meshes" folder! Where do I put all of my NIF files?

Well they have to go in Data/Meshes.

On a clean install you won't have a Meshes folder by default.

When you open the compressed Deadly Clutter file you downloaded, extract the meshes folder right into your Data folder. That will essentially create the Meshes folder you need for any new mods that include new meshes.

Example of extracting to Data (be sure you navigate to where your data drive is on your computer, mine is not in the default Program Files/Bethesda.. installation directory.)