download the "halftone191.zip"
extract the folder, and double click on the "Halftone.8bf" file. That should add it to the filter list in photoshop.
To make patterns with it, make a new document at a large size, fill in with around the same gray color that you need, then use the filter and play around with the lines/inch and angle. I think the angle for most standard textures in manga is the default 45, but sometimes you might want to mess with it. So after you use the filter, you'll have a bunch of pure black dots, and then you need to resize the page down until it's about the same as what you're trying to match in the manga.