How can I view replays?

Depending on your OS, the AutoRecord folder to place replays in is located in:

Vista:
C:\Users\username\AppData\Local\Ironclad Games\Sins of a Solar Empire\

XP:
C:\Documents and Settings\username\Local Settings\Application Data\Ironclad Games\Sins of a Solar Empire\

Just drop the file in there, launch the game, click Single Player > Watch and then select single/multi based on where you put the replay file. Note that you might need to toggle "Show hidden" in explorer as the path is hidden by default.

Annatar, thx indeed!

OK, so i've put the replay file in the above-described directory, in the Record-singleplayer subfolder. (The file name is: leinad_vs_hunting.record, I downloaded it from this site.)

When I enter the Watch menu, it appears on the list, but says "Invalid save file", and I can't go on. What should I do now?

edit: I tried in the multiplayer subfolder as well, got the same error message.

That means that the replay is from the older game version, unfortunately. Replays, like save games, usually are not compatible between different patches since changes to the game mean that the actions would not happen the way they were supposed to.

Nothing can be done about it, sadly.