At first sight I'd say there's nothing wrong happening here in relation to your model or the render process.
I missed your remark on the diffuse setting. At first I read it like you meant that upping diffuse didn't work, but now I see you meant you can't find it.
If you go to your model's node by clicking on the "Objects" bookmark in the node network, you'll see at the right side of the node a plus-symbol (+).
Double click on it.
If it's a population of your model then you'll see another grey node, which is the object holder/node for the population, also double click on the plus-symbol of that.
After doing that you are inside the internal node network of the object and you should see a bunch of red shader nodes.
If it's a single object then you should already see the red shader nodes.
Click on a red shader node, say for skin, bark or leaf and you'll see a "diffuse colour" setting.
In many cases it's default @ 0.5.
If you increase that then your texture will brighten up.
For starters compare default 0.5 with 1 and see how much you like that, then start tweaking from there.
Good luck!
