Ok, I notice your issue, but you can do this.
1) Your computer is offline. No internet connection.
2) Fill up your product key and Email.
3) Click Activate.
4) Since you have no internet connection. You will get this pop up message :
5) Click the Offline Activation button and you will get the save activation request pop up.
6) Choose where you want to save the Start10.req file.
7) Then you will get this pop up
8) Save the file into USB drive, bring it to computers with internet connection.
9) Upload that Start10.req file to http://activate.stardock.com/ to get your ".resp" file. Save that file.
10) Bring it back to your original PC and activate it by selecting that "Select offline file"
Hope that help you.
Stardock Community Assistant