parts can only hold bodys. That means that old freecad projects which don't have the body feature can not be put into a part. Jan wrote a conversion tool for that purpose, but that has not been merged yet. If you have a body outside a part and want to add it to a part, you currently need to use the python console. Look at the output of the gui "add part" command to see how it's done.
Jan wrote also a nice tutorial on how to use the new part design features, have a look at http://freecadweb.org/wiki/index.php?ti ... Tutorial_I
. If you double click the body inside the part you can model anything you want, it will be grouped beneath the body feature (thats the purpose of it).
To assemble stuff you need multiple parts. than you can select any geometrie on two different parts (maybe two faces) and hit the constraint button (e.g. coincident). I started writing a tutorial, but it's not exceeded the theoretical basics, so not too useful up to now. http://www.freecadweb.org/wiki/index.ph ... c_Tutorial
Just remember it's draft, pictures are inconsitent etc.
Because of the mentioned shortcommings the workbench is not realy easy usable up to now, but if you still go on testing I would like to hear you opinion about the constraints and solver part. Dos it work as expected? Dos anything solve as it should?