In Sublime 2, in order to call up the build results window you can use the Tools > Build Results > Show Build Results
menu item.
If you're using Sublime 3, the same menu item exists but is also bound to a key as well (check the menu item to see what it is for your platform), and you can click on the panel chooser icon in the bottom left of the window to open a menu that displays all panels and choose the output panel from there.
If you're using one of the beta builds for Sublime 4, the same options exist as for Sublime 3 but you have to right click on the panel chooser icon to open the panel menu, as a left click instead toggles the state of the side bar.
Regardless of the version of Sublime that you're using, the build results can only be displayed in a window where at least one build has been performed (each window has its own panel). For Sublime 3 and 4 that means that the panel won't appear in the panel chooser for windows that haven't completed a build yet.