Hello Don,
The pm [personal message] system is to send & receive messages off forum [so you can exchange email addresses & such like which are not allowed on posts so as to avoid spam]. You can also use it to inform of names of living people, another no-no.
The personal message system is open to you now you have made 3 posts, so you should receive notification when there is a pm for you. Along the top of the screen, under the RootsChat banner you will see a brown bar - the 6th option across is 'My Messages' and you can click it when there is a message for you. Once you have David's email address, though, you can exchange email in the normal way.
If you want to send a pm to David containing your email address, click on the text icon under his name on one of his posts, alternatively click on h is name & one option with be 'Send PM'.
Good luck....
Josey