Many online book-stores add copy protection to their books, commonly referred to as DRM or Digital Rights Management. If you have purchased a book through one of these stores, chances are that they have locked you into only using their software for reading.
ReadMe! only works with DRM-free books, that is books that do not have this copy protection embedded in the book, and the book format must be ".epub".
It's important that you know about the restrictions in the books you currently have, so you should first find out how to check if your books have DRM embedded in them.
Do a search on Google for "drm epub" to find out what steps you need to take.
(or click on this link: https://www.google.com/search?q=drm+epub )
Some other book readers use another format than .epub, and since ReadMe! currently only works with .epub it won't work straight out of the box. The best option is always to find the book in the .epub format, but this isn't always possible so what you can do is to convert it to .epub using one of the many services out there that does just that.
We have no affiliation with such a service, but we have tried this service called www.2epub.com a couple of times and it has worked pretty well. The chapter information was completely lost in the conversion so it's not as convenient to navigate the converted book as a native .epub-book, but the important thing is that you'll be able to convert it and then import the book to ReadMe!.
Note that we are not able to answer any questions about their service but can only suggest that you try one of the many other services out there.
To learn more about reading other formats than the .epub, click here.