There are two options.
1. You can click on the name of the user you want to PM, it will open a little drop down menu and the second option is "Send a Private Message to ...". You can click that and it will bring you to the PM window where you can type your message, etc.
2. On the top/right side of the page where you see "Welcome, Sherri". Right under that you'll see Private Messages. Click on it and it will take you to your user CP page. On the left side, you will see a list of links. Look for the one that says Private messages, right under it, you will see Send New Message. Click there, and it will open yet another window. There you can type the name of the person you want to PM and then the body of the message.
Peluito's King Bumi (Bumi)
Across the Rainbow Bridge - Starborn Mi Pequeño Tesoro (Toby) - Miss you my sweet boy.