Keyboard shortcut to switch between applications in Mission Control
There is no keyboard shortcut to switch between applications within Mission Control.

In System Preferences > Keyboard > Shortcuts > Mission Control you can see all of the built in keyboard shortcuts available:


Mission Control
Show Notification Center
Turn Do Not Disturb On/Off
Application Windows
Show Desktop
Show Dashboard
Mission Control

Move left a space
Move right a space
Switch to Desktop n



None of which allow you to move between applications in a space. 

Mission Control is designed to be used with a mouse or trackpad. If you only use the keyboard, don't activate Mission Control, instead use command + tab to switch between apps.