IF they've made their email available:

On the person's post, go to the icons to the right of the posted date.

If you click on the second symbol, the one that looks like an addressed envelope, your email program will automatically be called up with the correct address for the person -- IF their email is available.

OR

You can click on the first symbol, the one with the question mark, and you'll see the person's profile. If their email is available, it will be listed there. You can copy and paste this into the "To" section of your email.

If their address isn't available, the only option is to ask for their email, or to post your email and ask them to email you.