Brush Palette:
Those are found in the following map editors only: StarForge and SCMDraft 2. They give you the possibility of saving your own blends to either share them with others and/or to have a fast-access to them.
To open the main brush menu, you must (example based off SCMDraft 2 editor):
1) Open your editor
2) Go in the menu "Windows" and select "Brush Palette"

And there you go, you are ready to use the Brush tool.
How to use it:

• If you want to import a brush that is ready (either done by yourself or by others):
  1) Press "Open File"
  2) Find the brush you want and open it (you can open more than one)
  
• If you want to create a new brush:
  1) Do your blend
  2) Select it
  3) Give it a name (at the left of the button "Add Brush")
  4) Click on Add Brush
  5) Once you're done creating your brush with all the blends you wanted, you must save your work: Press "Save File" and choose a name for your brush (keep in mind you can always add blends to a brush, brushes can be modified once saved)

As references for extended/custom terrain, you must have these two things in your computer:

• Falkoner's Terrain Compilation
  
• WoodenFire's Crescent Dyne RPG map (which is included in Falkoner's Terrain Compilation, but it is such a big project that it deserves to be stressed out)