Usually (well for me) you have to watch the replay over lnce then you can start speeding it up and slowing it down. Did ya wait for it to end first?
^ this.
Unless you watched the replay getting made via single player or watching the match for a multiplayer replay you have to let it end once or twice if you pick the replay from the menu (or downloaded).
If you're still not sure. Look at the bottom right of TB, if the speed is grey, then it's not usable because the replay hasn't been "loaded" yet. so the < and > keys won't do anything.
Once the numbers turn black, your < and > will work fine.
"I like to visualise icky in the astral realm fighting off the spambots (alien-robot hybrid species) singlehandedly and trying to scan their brains to find out which evil mastermind is sending them" ~ rola
You can't slow down a replay or speed it up until you watch the whole thing. But don't just watch the whole thing and leave and come back to the replay later and try to speed it up. It has to be done at the same time that you watched the whole replay.