How do I use my Macbook Pro as an external monitor for my iMac 5K?

Hello,

I ran into a problem today. I have this devices:

• iMac 5K (max model)
• MacBook Pro (mid 2012) (512 MB Intel HD Graphics 4000) with OS X 10.8.5

The reason for the OS X version of my MBP is the speed. Yosemite just does not run well on it...

Well, that's that. I would like to connect my iMac 5K to my Macbook Pro. I would like to use my MacBook Pro as a external monitor for my iMac 5K.

The lady in the Apple Store (iCenter) told me it would be solved with a thunderbolt cable. Well, I read that too, but it doesn't. I connected the thunderbolt cable to both devices but nothing happens. I tried pressing CMD + F2 on my MBP and my iMac but nothing seems to work.

Could you please help me to get this work?

Thanks in advance.

Kind regards,

Lars Jansen

The Netherlands