The best way to check out the terminals on any pot is to use the ohm-meter function of your DMM. If you don't own a meter that should be your very next purchase before any other electronic item, it's essential for working with an arduino project.
Anyway with a meter if you test with a meter, two of the terminals will read a constant ohm value between them no matter where you move the slider. The third terminal will read a variable resistance between itself and either of the other two terminals. Usage is normally to wire one of the 'end' terminals to arduino ground, the other 'end' terminal to +5vdc and then the wiper terminal to a analog input pin.
Here is a picture of a linear slide pot. Note the two 'end' terminals and one next to the left end terminal. In this example the end terminal on the left would be grounded, the far end terminal to +5vdc and the 'wiper' terminal is the analog output voltage to be wired to a arduino analog input pin.http://www.allelectronics.com/make-a-store/item/SLP-50KS/50K-LINEAR-SLIDE-POT-CENTER-DETENT/1.html
Is that helpful? If not if you could post a picture or a link to the datasheet we can try some more to help you figure it out.