ADE - trying to add an ebook for offline reading

I want to read an ebook offline and this is where I am having problems. Its not my ebook but a library loan ebook, using cloud library. I was advised to try and add the ebook file to ADE via under ADE file - add to library - but all I can see is other files i.e. chrome and not the acsm file. I've searched everywhere for the acsm file but it won't show up. I went this route because if try to read offline - it asks for an app and ADE is not listed/won't show. Its a PC with windows 10. Any thoughts as to how I can fix this? or where I am going wrong? Thanks!