Thanks for your question. You should be able to click on your Avatar or community name once you are signed in and it will bring you to your profile. There will be several tabs on top, click the one that says private messages.
Sprint Social Care
I am another customer in need of assistance to but can't get anywhere trying to go exact same thing but apparently my phone doesn't allow for private messages to sprint care customer service, or sprint chat none of those options show up. Yes I am signed in, have been trying to get assistance all day, everything I have been told to do has led to nothing. I need to talk to someone immediately, ASAP, I have sent over dozen messages today please please help.
They are extremely difficult to find and the Sprint site is super confusing. The best way is to use this is to first sign in to the Sprint site, then use this link: http://bit.ly/1qQty1X. Next, click on the blue "Private Message" link. Then click "cancel" at the bottom on the email screen. This will take you to a page showing your sent and received messages on the right side.