Q: connect imac 2008 to macbook 2013

If you just want to transfer some files use the network, share them on the Mac that holds the files then connect to that machine from the recipient machine.

http://support.apple.com/kb/HT1549

ludovic,

Connect them for what purpose?

Sorry but your 2008 iMac is simply too old. There is a software tool however I doubt you would be satisfied with the performance. What you are attempting to do is called Target Display Mode, in order to use your MBA you would need an iMac that had Thunderbolt ports, which would be a 2011 - 2013 model.

The software tool I alluded to is called ScreenRecycler, you can find it by Googling the name.

On TDM, you may be interested in reading Apple's FAQ, located at http://support.apple.com/kb/ht3924

Your welcome.