When adding a persona to an account in Zimbra, and accessing it via an Outlook 2010 account, the Presona does not show up in the From: selection list.
(Where you select the account to send from)

Other posts in the forum refer to going to the Tools>Option>Persona menu in Outlook and selecting the "APPLY" button, however this does not exist in Outlook 2010.
Going to the Zimbra tab in Outlook and selecting Persona showed all the correct personas, however there is not an apply button.

Solution:

To get Personas configured in the Zimbra platform to show-up and be available to use in Outlook 2010 you need to do the following.

In Outlook,

Select the File tab
Select Options
A Dialog box will open
Select Add-Ins
In the main panel, look for the "Zimbra Sharing AddIn"
Select/Highlight the "Zimbra Sharing AddIn"
At the Top of the Dialog box, Select the Add-Ins Option button
This Should open the Zimbra Add In dialog box.
Select the Apply button
Select the OK button

All going well, your personas will now appear in the From: selection list.