How can I use my Imac Snow Leopard Disks in to my Macbook?

You can't use the disks from another machine. That is why you are getting that warning.

To upgrade the Macbook you need to buy retail version Snow Leopard installer discs.

Migration Assisstant is telling you that the software you are trying to move is newer than the operating system on the Macbook and won't work.

It´s illegal or has to do with license matters??

Kind of one and the same in the case of the included discs. First, any license is one-disc-one-machine, except the Family Packs. The included discs (which may include bundled software) are considered part of the complete computer package to give you a fully working computer vs. a box of hardware, not really a separate component. They may also include software that is specific to maintaining and diagnosing issues on that particular model, so those discs should be included with that computer if you sell it again.

Because it doesn't have the necessary hardware drivers for the MB.

And it's illegal.

The disks that ship with each Mac are system restore disks, not full retail versions of the OS. The distinction is that the disks that ship with a Mac are specific to the particular model of Mac that the disk was packed with. The disks carry only drivers for the hardware of that model, and will only install on that model of Mac.

If you would like to upgrade a 10.5 system to 10.6, you must purchase the OS X upgrade. In the US, it retails for $29 and can be purchased through the online Apple store. This will be the only way to upgrade your system.

Also, you probably don't want to use migration assistant unless you are truly migrating from one system to another. Generally speaking, it's faster and safer to simply copy files from one system to another.

But if I want to erase the disk and install from scratch, using the the disk from another machine, this can be done???

No. Others have already posted with details why this technically and legally can't be done so I'll let you read those. 🙂