Most build orders are matchup-specific. However, on the ladder, you don't know your matchup until you see the loading screen, and by then, you only have seconds to pick the build order. To make it easier to quickly pick build orders, you can bind build orders to hotkeys to open them instantly.
1. Make sure you are logged in.
If you aren't logged in, you can click the gear in the upper-right of the window.
2. Open up a build order and click on the red pin.
3. Specify what matchup you want to use it for.
An alert will appear to type in the matchup (e.g. "ZvZ"). It should be pre-populated with the right matchup, but you are welcome to change the matchup. Make sure you enter it in the form "XvY", where X and Y are the first letter of a race.
4. To check that it was propertly set, click on the gear in the upper-right to open up "Account Settings."
The matchup should no longer be grayed out. It will also provide the hotkey for you to use to open the build.
5. On the loading screen, use the hotkey combination (e.g. Ctrl-Shift-4) to open the build order.
Note that it may take a few seconds since your computer is already working hard loading game content into memory and configuring the game. As soon as it's ready, lock the build order window (probably Alt-Q) and head into the game!