I screwed up choice of email app

It has been a long time since I used UBUNTU and I am loading the 15.04 system for a dear friend on a new computer.
Seems my learning curve wraps around distant planets but anyway.
I need to convert an Outlook pst file to the mbox format so I can transfer it to the UBUNTU system - but I am easily led astray and installed Mozilla Thunderbird for the good or the bad?
Cannot for the life of me see how to load a contact list - maybe POSTFIX would have been better??
Please could someone help me sort out this glitch as I have no idea what I am doing.
I also have no idea how to enter line commands.

When this thing works it should have her contact list installed - if I have to migrate to another email app - how do I do that?

Thanks things will look better when I get some calm advice.
Mike