File > Properties > Drawing Model
On the initial 'Recource Centre' web page which displays when ProE first
starts up you'll find a tool called 'menu mapper' - as it suggests it maps
commands from the old  interface to the new interface - very useful.
Based on the way i read your question. Go to the browser in the drawing and
right click on the assembly or part you would like to add,
for this example, lets call the assembly you want to add as SUB-ASSEMBLY.
when you right click on "sub-assembly" a menu will appear and look for "Add
Ok Now, lets assume the name of your main drawing is called ASSEMBLY_1
Now, go to your main drawing window and look at the bottom of the screen in
that window for NAME: ASSEMBLY _1 once you see that, double left click on
and a window will open up...look for SUB-ASSEMBLY and now that is activated.
when you insert drawing views now. it will only pretain to that assembly.
hope this helps.