An .acsm file is a like a link pointing to where the real ePub or PDF is on the server.
You'll need to download
Adobe Digital Editions and register an Adobe ID (so you can later transfer your downloaded books to up to 6 devices). Then you double-click or drag the .acsm file to the Adobe Digital Editions app, and it will download an ePub or PDF that you can transfer to your Kobo.
I think you also have to register your Kobo with the ADE app, probably by plugging it in and waiting for ADE to auto-detect it. The details are somewhere in the Kobo User Manual.
Hope this helps, and welcome to MobileRead!