Assign textures

You can drag a texture files to the objects, Material Editor, Modify tab and viewport.

Drag to the objects or Material Editor:

This will create a new material* and apply it to objects or place it to the Material Editor (Standart or Slate).
Also you can drag/assign a texture to selected face(s).

Drag to the Modify tab:

Example for Displace modifier.
*If more than one textures is selected, material will be converted to one multi-material.
You can specify a type of created material from "TextureMap Options" (list of supported material depends on installed renderer plugins)
After merging all your options for merge will be applied.