how do i make knex online?

ive heard of knexbuilder but i cant find it plz help

There is no K'nex digital design software.

From what i've read most people who a really into building K'nex creations use CAD software and create there own CAD files for each part so they have full control over there parts library and how they can manipulate each part into a build. Any standalone software focused on K'nex building will be very limited in what you can do with the parts and will be slow to update if any new pieces are released.