According to the instructions on the website I should be able to got to the Line menu, click on Settings, and the register my email address. Massive problem: Settings is grayed out, non-functional, and masses of searching the internet seem to indicated that no one else is having this problem. Does anyone have an answer? (it will help you to know that I do not yet have a compatible smart phone...)
I'm afraid that you won't be able to use it, as for using the Computer Version you need to have previously registered your email on the smartphone version. This is stated on the login screen of the Computer Version.
As a side note, Settings is grayed out because you have no account to configure.