Here is how I've been accessing it, together with the issue and a suggested fix for it:
Click on "User Control Panel" in the menu bar.
Mouse over the "Overview" button-link in the UCP and click on "Private messages" in the drop-down menu.
I get taken to the "Compose message" sheet; there does not seem to be a way to get directly to my Inbox instead. It strikes me as though your Inbox would be a more logical place to get taken to in the first instance.
To get to my Inbox, I need to mouse over the "Compose message" button-link and then choose "Inbox."
This is somewhat counter-intuitive and a little confusing, also a bit annoying that it's easier to compose a new message than it is to get to your inbox.
Would it be possible to make it easier to access the Inbox?
Here is what I'd suggest, if this is possible:
a) In the "Overview" menu on the UCP, have two entries where there is currently one for "Private messages." One for "Inbox" and another for "Compose PM"
That way, the Inbox is easier to access without any loss of existing functionality.
b) In the "Private messages" menu-button link, have two options, one to allow you to go back to the Inbox and another to compose a new message.
Or is it just me doing something wrong?