1) Select the "Member CP" menu item from the horizontal green menu bar at the top of the forums (left most menu item).
2) On Member CP, select "Edit Options" from the vertical menu bar on the left. It is the 4th item on the third menu group down.
3) On the Edit Options screen, there is a group of options titled "Messages & Notifications". Within that group, there is a group titled "Private Messages" with a number of check-boxes.
Now we get into opinion. In my opinion, what you want in the very least is to check the boxes for :
"Receive Email Notification of New Private Messages" and
"Save a copy of sent messages in my Sent Items folder by default"
The first option will send you an email at your own personal email account whenever you receive a new PM
The second option will keep a record of the messages you have sent.
Don't mean to talk down to you, you said to keep it simple. Talking about menu items on a piece of software can get confusing, writing about it makes it even more confusing.
If it seems like this stuff is hard, don't sweat it. In no time you'll be skipping around and poking through the stuff like you been doing it all your life. It just takes a little while to get oriented.
Now then, when you are making a reply or sending a PM to someone, there is a group of options at the bottom of the screen - three check-boxes:
"Request a read receipt for this message"
"Save a copy of this message in your Sent Items folder."
"Automatically parse links in text"
If you set the options recommended above, the 2nd option should already be checked.
The first option SHOULD send you notification when they ready your PM. I say SHOULD because I have never used this option before. I don't know if the addressee has to agree with sending the notification or what. May creep them out and they'll think you're stalking them or something
At any rate, it sometimes seems to take a while to get responses. Sometimes folks just don't stop by as frequently as you'd think. Sometimes they are uncomfortable writing and want to chew over a response before they commit to typing. I dunno - a million reasons, I guess.
If this doesn't help, or you are still having trouble, just PM me