CS5 doesn't get updates any more. Only the latest version (CS6) will be updated.
Fortunately, the A77 isn't that new, and there was an update for CS5 for the A77. So, all you need is the last Camera Raw update for CS5, which was 6.7.
You can get it using Help > Updates, in the CS5 menu.
You can also download it directly from Adobe, at: