How can I get my new receiver to decode the AC-3 s

I believe you need an RF demodulator between the AC-3 out from the LD player and the input to the receiver. Three that are fairly reasonably priced, used, are sometimes available on ebay. They are:

Yamaha APD-1
Pioneer RFD-1
Kenwood DEM-999D

The Yamaha is probably the first choice. They sell for around $120-$140.

Dolby Digital is AC-3 (and vise versa). I agree with Ncarv that the problem is not the receiver, but probably the Laserdisc player RF signal is not the same as a DVD player signal and therefore not compatible.

Are you sure your Laserdisc player will read Dolby Digital?

I will check with a videophile friend who knows a lot about laserdiscs. He has owned all the variations in laserdisc technology going back to the original Pioneer analog players. I have his old first generation digital laser player, and I've never tried to run it other than in Pro-logic.

The previous two posts have correctly identified the problem as being the RF modulator. If your receiver does not have an RF-modulator input jack for laser disk players (and most new units these days won't, since laser disks are "old technology"), then you will have to buy one and insert it between the laser disk player and your receiver. Laser disks used an analog recording format for the audio signal, not Dolby Digital (or other digital surround medium), which is why the audio signal must be converted to a radio frequency that can be utilized by the receiver or surround processor.