The portable installation is designed to be an auxiliary install that you can carry around with you. To set up a portable installation, you need to have one computer with a "standard install" with your License enabled, which is used to create and update the portable installation.
It will be challenging to use MC's Portable mode as your only installation. Each time you want to update the portable install, you'll need to (I believe, at least officially):
1. Install it normally on a PC.
2. Install your License on that PC (if this was removed or never licensed there before).
3. Use that PC to create/update the portable installation.
4. Uninstall the copy from the "regular" PC.
If you have a computer at home, you'd be best installing your copy there, and using it to create the portable copy (which you use at work, and whatnot). That's how it is designed to work, and otherwise you'll be "fighting the system".