TotalBiscuit has a series called I Suck at Starcraft where he comments on his own noob games, focusing on what mistakes were made and how to improve.
The series starts out very basic, explaining economy, supply, teching, and the uses of some individual units. It should be understandable even for someone brand-new to SC2. The downside to this is that it may be too basic for many players. However, over the course of about 15 videos, he moves into (somewhat) more advanced concepts.
In your "recent" replays folder in game, there is a button to the left called "Keep". Pressing this button will move the replay into the "Multiplayer" tab if it was a ladder or custom game.
Then, from here, navigate to your SC2 account directory, which on Windows is usually located under:
C:\Users\Username\Documents\StarCraft II\Accounts\Some Numbers Here\Some More Numbers Here\Replays\Multiplayer
Also, you can download replays from the internet and then copy them directly to that folder, and they should show up instantly under the "Multiplayer" replay tab in the game!
In addition, any of the replays that you have not selected to "Keep" should show up under the "Unsaved" folder within your account under "Replays", here:
C:\Users\Username\Documents\StarCraft II\Accounts\Some Numbers Here\Some More Numbers Here\Replays\Unsaved
That folder will only keep about 25 of the most recent replays before it starts replacing the old ones, so please refer to this question for more options on how to save more, or manage your replay saves.
Best Answer
It may be that the replay is from a previous patch, which can cause this issue.
You can view the replay by placing it in the replays folder located here:
%username%\Documents\StarCraft II\Accounts\xxxxxxxx\x-xx-x-xxxxxxx\Replays\Multiplayer
There should be a shortcut in the
StarCraft II
folder pointing to your account.You can then access the replay in-game.
StarCraft II will load into the previous patch and play the replay. Note that you will be offline on Battle.net while viewing an old replay.