Switch to the "Vertex-Creating" mode and create three vertices in the front view.
Then klick on the "Triangle-Creating" mode and click on every vertex once.
Now you should see the triangle in the 3D-View. If not you should click on the "Triangle-selection up"
button and then on the "flip triangle" button. The invisible-side changes to the visible-side.

Click on the "select vertex" button and click on one of the 2D-views(So it get's the focus).
Then use the arrow keys ;) .

Click on the "set texturecoords" button. Now you can see a window, with the texture you loaded.
If you want to move the coords now, set new values in the editable fields (0.0-1.0).

The window looks like this: