1) Well, yes, you need to give it the database path. When you got Rosetta, you either got it as a tarball that included a rosetta_database subfolder, or you had a link to the academic downloads site which has just the database as its own tarball. Find where you put that and pass the path to the database flag. If you put it at /home/myself/rosetta_database, then “-database /home/myself/rosetta_database” will do it. Make sure you have write access to wherever you put it; for some operations it generates binary versions of text files as a speed optimization; this requires you to be able to write the new version of the file.
2) You need a database (path) because we avoided hard-coding energy function and chemical data into Rosetta. Rosetta’s code doesn’t know what alanine is, but it knows to read the database automatically, where it will find alanine when it needs it. Rosetta’s code doesn’t have the entire Dunbrack rotamer library coded in, it just reads those values from a database file if you need to to repacking.