Used stamps exchanges.

I'm new here and I don't know how to get an ID when a memeber sends me stamps and I want to repost to him.
So the question is: how will I get that ID?

The firtst time I only wrote my name on the enveloppe, so the receiver does know I sent him stamps but it's not registered

Arjen,
this club is not ME-->YOU and YOU-->ME
it is random...
I received your letter and I registered it. I am not obligated to send you stamps back
I go on page and make a demand for trading partner - could be you or any other participant
(and guess what - it was you - but it is more coincidance, than anything else)

I got an enveloppe today with only the number of the sender.....how can I find the adress?
I would like to send stamps back.