iPhone
I am meant downloading pictures from my iPhone to my Mac not from my camera to my Mac. I accidentally hit don’t trust and now I don’t know how to allow it to trust again without messing up all other settings.
MacBook Pro 15″, OS X 10.11
in theory I believe it's suppose to prompt you again when you disconnect and reconnect the device.
here is a support article on the subject. there is a section called 'Get help' that will hopefully help out
About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support