Managing multiple Office 365 Accounts? Can't purchase new Personal Plan!

I had an Office 365 account for personal use on my iMac (mid-2011). My employer recently signed me up for a business Office 365 account.

My business Office 365 account doesn't allow editing (any Office docs) on my iMac. I'm not entirely sure why but that is the message I get when I try to do that. Up until now, I've just been using my personal Office account to create, edit docs and that hasn't been a problem.

Yesterday I opened up a document and found I couldn't edit it. Assuming my personal account hadn't been renewed, I tried to buy a new personal plan. Every time I try to purchase the plan, I am redirected to my Business Office 365 account.

Gah! Help! How do I manage multiple Office 365 accounts? Why can't I edit on an iMac with a business account? My employer says it has nothing to do with Administrator controls.

Any enlightenment most appreciated!