The only time you will see + or - on your digital meter is when you are measuring DC voltage. In your case, you are measuring resistance ( Ohms ) . When you touch your meter probes to the speaker wires using the Ohms Scale, you are putting a small DC Voltage thru your meter probes, it's how meters work when using the Ohms scale. A small current will now flow thru your speaker pushing the cone in or out. Which way the cone moves ( in or out ) depends on how you have your meter connected to your speaker wire. This is basic electronics. ( Ohms Law ) Keep trying and it will hit you.
If that meter was sitting in a garage for a while, you might want to check the battery's.
If you want to see + or - on your meter, put your meter on the DC Scale, go out to your car and touch the probes to the car battery. Touch the posts one way and then the opposite.
(LCD - Sony KDL -52 XBR4) (Receiver - Yamaha RX-A1040)(Blu Ray - Oppo BDP-83) (PS3)( Comcast X1) Speakers (L & R - Paradigm Studio 20) (Center -Paradigm CC-470) (Surrounds & Back Surrounds - Paradigm SA-15R in walls) (Subwoofer 1 - Sunfire HRS-12) (Subwoofer 2 - Paradigm PW-2100)