Hello, Zographos. I see this is your first post. Welcome to the forums.
The first thing I'll mention to you is be careful in your terminology. The 13 elements that those RAW files contain are channels, not layers. There's a huge difference between the two. The file will only have one layer, and 13 channels. Do not try to add more layers.
You can find the channels listed on the Channels Palette. Before I tell you where to find that palette, let's make sure you've got it in a standard place. Go Window -> Workspace -> Default Workspace, and then Window -> Workspace -> Reset palette locations. That will ensure that you're looking at a "normal" Photoshop interface layout.
Okay, now look in the lower right hand corner of the screen. You'll see a window with three tabs at the top, labeled "Layers", "Cannels", and "Paths". Click on the Channels tab, and you'll be looking at the Channels Palette.
You'll see all the channels in your image, arranged in a list. If you're looking at the RAW file you downloaded from your sim, you should see all 13 channels listed right there. If you're starting from scratch, then chances are you just see three channels to start with, Red, Green, and Blue, as well as a composite of the three, labeled RGB. So you know, RGB is not actually a channel; it's as I said, just a composite of the three individual Red, Green, and Blue channels, so ignore it.
To create additional channels, locate the "Create a new channel" button at the bottom of the Channels Palette. It's the second one from the right, the one that looks like a square sticker being peeled from its backing. Every time you click on it, you'll add a channel to your image. (Be aware that that same button on the Layers Palette adds layers, so make sure you are indeed working on the Channels Palette. You don't want more than one layer in your RAW file.)
Now, to select a channel to work on, simply click once on its name in the Channels Palette to "activate" it. You'll see that the active channel's name gets highlighted, and that visibility gets turned off for all the other channels. (Note: the little eyeball icon indicates visibility; to turn visibility on or off for a channel without activating it, simply click the eyeball.)
To find out what all the various channels do, read the RAW terrain file instructions in the knowledge base. You can find them either by Googling for "Second Life RAW Terrain" or by clicking
this link.
If any part of the instructions is unclear to you, take it as a sign that you probably need more experience with Photoshop before you proceed. At the very least, you're going to need to know how to assign grayscale values to colors, and how to paint. The more you know about image manipulation in addition to those basics, the more success you'll have. (And of course, the less you know, the more you'll flounder.)
If you're new to Photoshop, I would not recommend trying to learn it with this particular project. The 13-channel application of RAW files that SL uses is pretty unique, and requires non-standard work flow. I'd highly recommend you learn Photoshop through more traditional uses so that your initially formed habits are universally applicable. Then, once you've got a good handle on how the program works, return to RAW terrain files so you can understand what habits you're setting aside. Remember, it's much harder to break a habit than to form one. Make sure the ones you form in the beginning are the right ones.
Good luck.
