You can't. It's free :-)
Or more precisely, I'd like AscToTab to be postcardware, which means it is free to use without conditions or limits, but if you continue to use it would be nice if you said a small "thank you" by sending me a picture postcard at
John A Fotheringham
c/o JafSoft Limited
35 Nursery Road
If you'd like to be informed of future updates, either keep coming back here, or better still send me a postcard to the above address with your email address on it, and I'll notify you of any updates (although these are now extremely rare).