Since the PS-WE551 did not ship with the F551, it won't be compatible, even though it's also a Samsung. You really need the sub that came w/ the F551. I could be totally wrong, but that's usually how it works with the proprietary systems like this.
Ask in the Samsung sub-forum too.
I have a Samsung Soundbar HW-F551 and a Samsung wireless Subwoofer PS-WE551, ive been trying to connect the two together without success, the more I read into it the more I think these two units are not compatible, is there any way round this or can I run them independently with wires, any help would be great, Cheers, Tony.