Where do I get the urllink.acsm file? I am unable to open books and understand that I need this file to designate Digital Editions as the app
Without more information, it's hard to give you the exact answer to your question but I think I can help.
First make sure you have Overdrive installed on whatever device you're using, or your computer if you are using a handheld eReader, like the Nook, and moving titles from your computer to the eReader. Overdrive is the app that manages library editions of our books and converts them (I believe from that ascm file) to the format Digital Editions is able to read. You can find and download Overdrive here.
Other than that, you may want to look at the step-by-step instructions for your device, which can be found here. Also note that there is a list of free programs at various library branches on the same page, where you can get one-on-one help with your eReader device.
Hopefully this will help you with your problem - thanks for your question.