Replays can be uploaded in .rec format or as an archive file (.zip, .rar) Placing your replay in an archive (with Winzip or Winrar, for example) reduces the file size significantly, making for faster attachment and downloads. All replays should be attached to your post using the attachment feature.
You can find your DoW2 replays in My Computer -> My Games -> Dawn of War 2 -> playback.
If you forgot to save a replay, your temp.rec file records the most recent game you played, so in some situations, you may be able to salvage the replay you were after if you forgot to save it as a replay ingame at the end of the match. Simply rename the temp.rec file to something else, and it should run as normal.
To view someone elses replay, just download their file, and place it in your own replays folder. It will then show up in your saved replays list ingame if the replay version matches the version of the game you are running.
Remember to tag your thread titles with the version number of the patch you played the game under.
example: '[1.2] Good game vs Orks'
Replays don't play correctly if you're trying to run one from a prior patch, so it's important you include this information in the title of your thread so that people don't waste time downloading replays they can't view.
In other news, the normal DoW2 section rules apply here.