1. Open up "Outlook"
NOTE: Depending on what version of Outlook you are using your layout may be slightly different.
2. Click the "Tools" option at the top of the outlook menu
Select the option labeled "Account Settings..." as is shown below
3. Selecting this will bring up this menu
Select the option labeled "Change" in the row of options above the name box in the center of the menu.
4. Selecting that will bring up this window
Click the button labeled "More Settings" in the bottom right hand corner.
5. Once you've clicked that, this menu should come up
Click on the tab labeled "Advanced"
6. The tab should look like this
Click the button that says "Add..."
7. When you select "Add..." this window will open
Type in the name of the person who's given you Access, then click "OK"
8. Once you've selected the person their name should now be listed in the "additional mailboxes" window.
Click the "Apply" button in the bottom right hand corner to apply this change.
9. Once that window disappears, click "Next" on the window you are now looking at, bringing you to this window
Click the "Finish" button in the bottom right hand corner.
In the left column under "All Mail Items" you should now see the persons mailbox listed (In this case, Nathan Smith). You can now select it.
If you have any questions regarding this issue or need assistance, please feel free to contact the IT department at firstname.lastname@example.org
If you click on the inbox and it says that you do not have permissions to view the inbox, please, contact the person who has granted you access and let them know that they need to give you permissions.