Se você quiser manter a tampa aberta, mas usar o monitor externo como a tela principal, vá até o painel Monitores nas Preferências do Sistema, clique em Organização e arraste o retângulo branco de uma tela para a outra.
Caso contrário, consulte este artigo, Usando um monitor externo com um MacBook , que diz:
When your laptop is open, you can't switch to using the full resolution of an external monitor. I have no idea why, but it's a fact of life for MacBook users. You can use both displays in "extended" mode, or you can mirror your MacBook's display (which most likely doesn't use the full resolution of your external monitor), but you can't switch to using only the external monitor. Very strange.
If you close your laptop, you can actually get the full resolution of your external display. Put your MacBook to sleep, connect your external display and an external monitor, and wake the computer up by pressing any key. You will have full use of your external display while your laptop is closed.
If you really want to have your MacBook open while only using an external display, after following the procedure above, you can open your MacBook, and its screen will remain off, which means you can still use the MacBook's keyboard and trackpad. Why you can't get into this mode using F7, I have no idea. This is a good technique for allowing some heat to dissipate while still using an external display so the lid of your MacBook doesn't melt or warp.