Can't Digitally Sign Excel 2013

So,

I imported an old 2003 xls file and have made modifications to the resulting xlsm, including updating commands to custom ribbon tab, etc.
I have long had my own Root Certificate and Publisher Certificate for my development jobs, (and other .net applications) and previously had signed the xls file with the publisher certificate. That did not import through to the 2013 xlsm.

So, after my updates I'm ready to finalize the project, and I go to the VBA Editor, Tools | Digital Signature.

I get the dialog, it says [no certificate] for both, so I click "Choose".
I get: No usable signing certificates are available. Please insert your smart card, or contact your administrator to obtain a signing certificate.

It wouldn't let me browse to a PFX or CER file on my hard drive, it just dies. It won't let me select of find my certificate with which to digitally sign my project.

Thanks
Jaeden "Sifo Dyas" al'Raec Ruiner

PS - Should be noted this is on a Windows 8.1 Home machine.

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"Never Trust a computer. Your brain is smarter than any micro-chip."
PS - Don't mark answers on other people's questions. There are such things as Vacations and Holidays which may reduce timely activity, and until the person asking the question can test your answer, it is not correct just because you think it is. Marking it correct for them often stops other people from even reading the question and possibly providing the real "correct" answer.