how can i put something under a null???

open the explorer, select and object an drag it in the null.

and what did this mean???????????
what should null be??

you should make your question more clear next time.
a null is an object in xsi, just like a polymesh or a curve. you can make one by choosing create->primitve>null
for future reference: if you press "F1" in xsi, the magnificent xsi-help opens up. it contains indormation to pretty much everything you can dow ith xsi and ha sa searchfunction

so this means i can only use objekts from the "null" categories in xsi if i want to make it to msh??

no (you poste that twice btw), it means that you have to drag everything you want to include in the msh on the null that acts as "root" and then export the null with everything thats under it. objects that arent under the root wont be exported and included in the msh.

so if i use the editor it musst look like this:

root
- cube
- cone
.
.
.

and not like this:

cube
root
-cone
.
.
.

if you want both the cube and the cone to be included in the msh, yes, if you only wnat to have the cone the second setup is right. (if the "-"s represent that these objects are child of(=under) the root)

jep ok thx