Installation URL identifies where your UseResponse will be located - in other words address in browser. You don't need to put the whole url there - just state your main domain (without sub-domains, folders or www).
To install it on local machine, just use localhost or update hosts file to have the part of installation url.
You can change url at My Account during 30 days period. If that period is not enough for you, please submit us a ticket and we'll do it for you.