Tiles are the main building blocks of the map. There are two types of tiles: floors and walls. They use different textures (though you can import the same texture under different types).

Tiles import is done by selecting "Import Textures" in "Import Resources" menu:

Under each table there are two buttons: the left one will remove current selected texture and the right one will open a file prompt, letting you select one or more textures to add.

After textures were added, press "Accept" and all your added tiles will be shown in "Tiles" tab.