They can work with each other. In order to pair them, I suggest you plug them in the same room or even in the same extension board which can save you lots of trouble. Try the steps below:
1. Press the pair button on the PA511 for 1-2 seconds, release the pair button, the power LED will begin to flash.
2. Press the pair button on PA251 for about 5 seconds and let it go, the power LED will also begin to flash after releasing the part button.
wait a minute, and see whether the middle LED will light up or not.
Hope this would help you:o

Have done all I can now.. just can't get it to work.

I read that the included cd have programs to reset the units so the pairing can be released.nus
Will this help me? Have not tried that.

/ Magnus

I just got a PA210 working with a PA551 by running the PA210 utility software and doing a factory default reset - that was the only way to get round it.