thanks for your post!
Yes, it is expected that you can't open a WP-Table Reloaded. The reason is that this is a very technical format, and there's simply no application that understands what the contents mean.
If you wanted to, you could open the file in a text editor (like TextWrangler), but everything will just look like garbage. But that's no problem, there's no real need to deal with the file directly. It's basically just for safe-keeping, in case you want to re-import things into WP-Table Reloaded or TablePress.
If you want "simple" table backups, i.e. in a format that can be read by other applications, I recommend to export the tables to CSV format. That can for example be opened by Numbers or Excel.
(In WP-Table Reloaded, this can be tedious, as you would have to export each table individually. If you migrate to TablePress first, you can then export all tables at once, to CSV files that are stored in a ZIP archive.)