MBP dual monitor set up

i'm trying to set up my MBP (with thunderbolt) to be the primary monitor and have my iMac (only mini display port, no thunderbolt) as the secondary. i used an HDMI to mini display adapter and it didn't work. My Display Settings didn't change and couldn't get the iMac to detect. i tried out Air Display to see if i can set up dual monitors. so the issue with using Air Display is my iMac can detect my MBP but my MBP cannot detect the iMac.

does anyone have their dual monitor set up with an iMac and a MBP and if so what are you using and how did get these two to detect each other?

 

i didn't know about the Command F2 so thank you.
well i connected everything again and of course the iMac is not detecting.

does anyone know if i can use a Thunderbolt cable from my MBP and plug the other end into the iMac's mini display port .. will that work?

 

okay what am i doing wrong again .. ?
bought the cable ..

Cable Matters Gold Plated Premium Mini DisplayPort Male to Male Cable in White 6 ft

connected it to both computers.

hit Command F2
tried Command F2 while holding function key

both won't trigger the target display mode ..

what am i missing ?

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the cable is connected to the mini display port on the imac and the thunderbolt display port on the mbp.

on the imac hit command F2 and then tried command function F2 .. nothing
opened display preferences. no target display options.

???