Accessibility access to control your computer is required to use many features including selecting menu items, manipulating windows, pressing buttons, typed string triggers and the window switcher.
You will need to enable Accessibility permissions for both Keyboard Maestro and Keyboard Maestro Engine.
On modern macOS, you enable Accessibility permissions by opening System Preferences, going to the Security & Privacy preferences, and in the Privacy pane select Accessibility and ensure that both Keyboard Maestro and Keyboard Maestro Engine are ticked.
⚠️ Note that for Mojave (Developer Beta 5 / Public Beta 4) you may need to relaunch the Keyboard Maestro Engine or make any change to your macros after enabling accessibility.
⚠️ Note that OS X Yosemite (10.10 GM1 through 10.10.2) has a serious problem adding applications to the accessibility pane properly. This bug was largely resolved in Yosemite 10.10.3. So if you are having problems with accessibility under Yosemite, upgrade to 10.10.3 or later, and then launch Keyboard Maestro.
If you continue to have problems with enabling Accessibility permissions, ensure you only have one copy of Keyboard Maestro on your Mac and restart and try again.
If you continue to have problems, contact support.
Historic Mojave Beta information which will be removed when Mojave is released
Note that OS X Mojave (Developer Betas 1-4 / Public Betas 1-3) has a serious problem adding accessibly for Keyboard Maestro Engine. Specifically after it is added, you can click the checkbox but it wont necessarily turn on. Resolve this by deleting the Keyboard Maestro Engine entry from the Accessibility list and relaunching Keyboard Maestro Engine (Keyboard Maestro File ➤ Quit Engine, File ➤ Launch Engine), and then you can check the checkbox to enable accessibility permissions.
Note that OS X Mojave (Developer Betas 1-4 / Public Betas 1-3) has a further serious issue in that even after it claims to have enabled accessibility permissions, the system may still be refusing to allow access to the event queue (symptoms include not being able to set hot keys and typed keystrokes not working (including Command-V for paste)). Restarting your Mac seems to resolve this.
Note that OS X Mojave Beta 4 (Developer Betas 4-6 / Public Betas 3-5) will not grant Keyboard Maestro Engine.app accessibility permissions while it is within the Keyboard Maestro.app bundle at all.