I'm going to assume that you are trying File > Open > Other User's Folder.
This dialog doesn't allow for the opening of those two. Now assuming that
permissions to those folders where granted by just right clicking on said
folder > properties > permissions tab.  Then the only way to open is where
UserA sends you an Outlook shortcut to those folders.  This is done by right
clicking on the outlook folder and drag it to the desktop, pick Copy Here.

Now there is a better way to have access to another persons mailbox.
Ideally in an Exchange 200x environment, you would grant yourself access to
the mailbox via Active Directory Users and Computers (ADUC) on the Exchange
server.  This is done via the Exchange Advanced tab > Mailbox rights.  All
you have to do is grant "Full Mailbox Access".

Once this is done, you can add that mailbox to your Outlook by right
clicking on "Mailbox - <username>" or "Outlook Today" folder and select
properties > advanced button > advanced tab > add button.