I've created the submovies that I intend to load into my main movie via loadMovie. I've saved them all in the correct folder, but when I test the main movie, I get the error Error opening URL "c:\windows\_desktop\somefolder\submovie.swf in the output window, and my movies don't load. What's going on?
Most likely, when you made your various submovies, you saved them correctly, but you needed to take the extra step of exporting them as .swf files (simply by using Test Movie). Only .swf files and .jpg files can be loaded using loadMovie.
My website is getting pretty messy with all the little submovies and.jpgfiles in the main folder. Is there a way I can keep the movies that load in a separate folder to keep everything straight?
Yes. When you specify the movie you want to load, if you simply type mymovie.swf, Flash will look for mymovie.swf in the same folder. If you want to store mymovie.swf in a subfolder called movies, you can change the URL field to read movies/mymovie.swf. (By the way, you can use all the standard HTML relative references as well, such as ../ to indicate "up one folder.")
My loaded movies appear down and to the right of the clip into which they load. Is the only way to resolve this to move my clip up and to the left to compensate?
No. What's happening is that the loaded movie is orienting its top-left corner in the registration point of the movie into which it's loading. It sounds like the clip into which the movies are loading used center registration (making the loaded clips look like they appear down and to the right because their top left is in the center). You can remake the clip and select the top-left registration point this time. Or you can edit the contents of the holder clip: Select everything and move it down to the right. Back on the Stage, you can move the clip up to the left.
If I know I'm going to use a shared Library item in a project, but I don't actually have the master media elements (such as the bitmaps I want to share), is there any way I can start now?
Sure. Bitmap items' Properties dialog boxes have two buttons Update and Import which let you reimport (and replace) a previously imported bitmap. The particular scenario of wanting to work without the final artwork is explored in a task coming up in Hour 23, involving authortime sharing (compared to this hour's runtime sharing examples). You might want to explore that task because it should help you fully understand the difference between authortime sharing and runtime sharing.