I am having trouble with the Digital Signature.

I want to Digitally Sign the package with our company name, but the instructions are vague. When I click "Digital Sign" it searches for a file on my computer, but I do not know where the file is. Could someone help me locate where the file for Digital Signatures is located?
This is the only software that I make, so I do not have additional software on my computer that I am aware of for creating Digital Signatures. I'd appreciate any help that could be provided.
Thanks,
Craig