It sounds like you have a bad keypad. There is nothing to set or reset other than the code itself. If it doesn't light up when you key 'enter' it is most likely bad or your battery is bad.

or the battery contacts are corroded/oxidized

I purchased & had a Craftsman 1/2 hp Model 139.53970SRT installed in 2002. My problem is when I program my new craftsman wireless keypad for ouside my remotes that came with the unit will not work. I can only use one or the other. Sears hardware personal are confused. My garage door opener has a orange learn button. prior to this I tryed both clicker & genie wireless keypad and the same happens. Thank You....

Sounds like you are getting the wrong keypad. Make sure the model number on your opener is correct and have Sears parts look it up for you.

I accidentally deleted all codes on my garage door opener when I setting up a new remote and now don't know how to set a password for keyless entry pad - I have no instruction manual since it was installed by previous owner.

Locate the smart/learn button on the rear of the motor unit near where all those white wires go. Press and immediately release this and then walk over to the keypad and key in any 4 digit code you like and press enter