Yes, I evidently misunderstood the way prestashop works!
I thought it was, for example, like having a Yahoo or Google account, that you can access from any computer that has the program installed.
Firstly, thank you for responding!
To respond to your question, I have two computers and I downloaded and installed prestashop in each computer.
When prompted for my userid I used the same email and password in each PC.
I am doing all the installation by myself.