To send a private message, click on the name of the person you want to contact and then you will see 'Send a private message to XXX'.
It's not a good idea to use this function for the purpose you mention. If you want your work to be checked it's best to post it on the board for everyone to see. That way you have the best chance of getting a speedy reply.
If you send it to me personally, for example, I might not see it for 24 hours or more and then may not have time to answer it for some hours more, even if I wanted to or felt able to.
Student or Learner