@GregX999
File names, models, and database tables have to have the same name. They only differ in case (and pluralization for database tables). Naming is explained in DataMapper Models.
They must be valid PHP class names, which means no spaces, only alphanumerics and an underscore, and must start with a letter. (While databases allow table names with spaces, I highly recommend against that in general practice; DMZ does not allow it at all.)
The name of the table must simply be the lowercase pluralized form of the class name. Therefore PressItem becomes pressitems, while Press_Item becomes press_items. The filename is simply the lowercase form of the class name: pressitem.php or press_item.php, respectively.
(Technically, the classname should only have the first letter capitalized, but PHP is case-insensitive for class names, so don’t worry about that.)
How you choose name multi-word items is up to you. I recommend using CamelCase and no underscores, as it makes debugging complex queries easier, since DMZ uses underscores to separate components when naming join tables and querying related tables.
Edit:
I forgot to mention that the name of relationships to the item, unless specifically set, is the lowercase form of the class name (pressitem or press_item). IE:
$foo = new Foo();
$foo->pressitem->get();