I am trying to assemble pictures in Photoshop 7. I render my characters , props and background scenes in Poser 5, When I create layers and bring the characters over to my Base image there are no shadows. Can anyone tell me how to get shadows with Photoshop. I also need to know how to use the Masks?  Thanks for any info.

masks are easy.
make a square or circle and fill it with black.
go to layer/layermask/reveal all.

look at your layer panel. see how a new square was added beside your original and how that square is all white?
make your foreground color black, grab a paintbrush at 100% opacity with a large tip (80 pixels), now paint over the black square or circle you made at the beginning. 

what should be happening is wherever your brush goes over the black it hides it from sight. if you accidentally hide too much make your foreground color white and paint over it again; thus revealing your black.
very handy as it isn't destructive to your pic.

all of the above would be reversed had we choosen:
layer/layermask/hide all

many other kinds of masks to choose from, this is just a basic example.
hope this helps.

When you say no shadows do, how do you mean, is it that there are no shadows for instance cast from a person onto a prop? I work in C4D and you use a compositing tag to stop certain things form rendering but still casting shadows. That said you could always just invent your own shadows on characters by doing a selection of a copy of a person for instance and filling it with black, then doing a distort to drag it onto the ground, then reducing it’s opacity to make it more grey. That is a very rushed synopsis and there are a lot of mags out there and books that would help you along better than I could.

I there are no shadow on the character render form poser have you enabled shadows in the rendering settings. Over in the poser forum you’d probably get people more familiar with poser output.