First, does this ebook have DRM?
If yes then getting it into Mantano is going to be a nuisance. Go to the Google Play Books website and try to download it. This link should work:
https://play.google.com/books
The file you get won't be an Epub file but more of a link file which will enable you to download the DRMed Epub file. You'll need Adobe DE to read that DRMed epub on your PC:
http://www.adobe.com/support/digital...downloads.html
I don't know specifically how to get it into Mantano, but I would start on the website:
http://www.mantano.com/mantano-reading-platform/