You should be able to find Animation Composer in "Window" -> "Animation Composer" menu.
If you have installed Animation Composer and it is missing in the Window menu try the following troubleshooting:
Possible solution 1.
Just restart After Effects.
There has been an issue when opening After Effects from Adobe Premiere caused the plugin to not load.
Possible solution 2.
Follow this uninstall guide to remove Animation Composer manually.
Then install it again.
Also make sure that you are launching the same version of After Effects that you have installed the plugin into. Sometimes installer does not list your version of After Effects during the installation step. If this happens you have to locate your After Effects installation manually with the "Browse" button.