Not sure exactly how you're doing it but try this.
Step 1: select plane and then right click. A menu will pop up, select Assign New Material, then Lambert. Now Lambert is applied to the plane.
Step 2: Click the option box (checkered) next to the color in the channel box. You should now see an empty field with a picture of a folder on a button next to it. Click on the button and find your file. Now the image is on the plane and again you should see it on the sphere as you did before.
Step 3: Put your cursor in the view screen and press the #6. You should be able to see it on the plane.

If the above steps don't work let us know and maybe someone else will have a better idea. P.S. I have Maya 8.5 on Windows so hopefully the above steps will work for you on Maya 7 and Mac.