can anyone tell me how to run "square up" on a computer?

Square needs to be online at the time of the "swipe". That is how you have people authorize the charge. Not doing so would be a violation of terms. To manually enter a number costs more. Square also runs on Android if you have a phone or tablet of that variety. To my knowledge there are not any legit iOS 3rd party emulators. Only what apple puts out for Devs. Paypal also rolled out a service like this, you might see if you can use it from the web.