I use a Windows platform for development. If I were to write a (brief) user guide for my app in MS Word, how would I connect it (or fold it into) to the app as a pop-up help screen? Making it a part of the app is preferable, because then it would work on a Mac.

Create a "Help" file and connect to it thru a Help button.

The Help file can be broken out into different layouts (I never use records, just text in the layouts, and disable creating/editing/deleting records); that way, you can get to different layouts in the Help file from different layouts in the main file. Additionally, you may want to add a navigation bar within the Help file to move from layout to layout.

My data input layouts can usually be divided in half either horizontally or vertically. I've put little help icons on these layouts so that a user can click and receive help while still entering data. I also built an entire help system with more detail using a FM file with a menu and scrolling layouts. I agree with Dan that you are better off staying and building using FM's features.