I cannot find Betas in STEAM directory.
You don't need any beta, now the game includes everything necessary. The mod loader was part of a beta version long ago, but everything that refers to a beta is obsolete now.
If you struggle with all the information given
there, you can simply look at the questions "How to access my game files?" and "I'm interested in a mod, how do I install it?" (See "for custom parts" for this one). You can ignore the rest.
I could not figure out how to "drop assets" into parts and textures folders.
The "assets" are simply a file with a ".pack" extension.
You need to download the file named "RDEP3.0.0.zip" from
this message, then uncompress it using an appropriate tool (7zip for example). You'll normally end up with a folder with a file named "RDEP3.0.0.pack" inside. This is the file you have to move into the following folder:
Spaceflight Simulator Game\Mods\Custom Assets\Parts
Once it's done, launch Spaceflight Simulator, and go into the "Mod Loader" menu. If you did everything correctly, the pack will appear in the mods list. Just activate it, restart the game, and voila
I tried to give as many details as possible, but don't hesitate if there's still something unclear.