Okay so… the diffference is bbpress & buddypress. So I used a bbpress shortcode. Ok ok. ^^
But if I just attach a page, you say, then a register form will automatically appear on that page? Or is there anything else I need to do first?
EDIT : I made a new blank page, attached it, & now it works.
So my final question & the topic can be closed:
–> Can I create/design a page & then “somehow” insert that same register form on my freshly designed page? 🙂 ( cuz now its just a blank-page with the form on it. )