Dear List,
After a few changes i have here an example:
It works, but what I wanted to know is why are there

round brackets after writing \starttheroem if i compile it.
Should be without round brackets.

I have the following example, the text was in a PDF from 2008, called: Theorems in ConTeXt, and also my text with the margin:


\definemargindata  [MyInMargin][inright]
\definemarginframed[MyInMargin][topframe=on,bottomframe=on,rulethickness=1pt,width=1cm]
\margintext{\blackrule[color=black, height=0.25ex, width=1cm]\\}\blackrule[color=black, height=0.25ex, width=7cm]
\setupenumerations
  [ before={\blank[big]},
    after={\blank[big]},
    location=serried,
    width=broad,
    distance=0.5em,
    headstyle=bold,
    titlestyle=normal,
    way=bytext,
    conversion=numbers]
\defineenumeration
  [theorem]
  [    text=Theorem,
      title=yes,
      style=italic,
       list=all,
   listtext={Theorem }]
\starttext
\margintext{2.1.1}{\starttheorem {The square on the hypotenuse is equal
to the sum of the squares on the other two sides. Ach is das schön, dass du da bist}
\stoptheorem
\stoptext

Maybe someone can help me.