I had the same trouble. Couldn't open the .RSD file. We had to do some study on Sea-Bottom profile. We used Garmin device to collect data off-shore. The folder with all the data was sent to us over email. The files were in .RSD format. I downloaded them but could not read them in Homeport. Turns out, that you have to copy the entire folder onto a memory stick. I did that. Then opened Homeport on my computer and a new Devices sidebar was added under My Collection with all the files I earlier could not view.
Long story short: Copy the entire folder onto a memory stick and then they become readable in Garmin Homeport.
Download and install Homeport software for free https://buy.garmin.com/en-GB/GB/p/64242#overview