How to model adhesive layers in Autodesk Inventor?

It really depends on what you are looking for...
I mean if it is just for visual purposes only you can use the extrude command or by making a surface the thickness command...
Straight forward procedure...

On the other hand If you are looking to simulate this model later on in FEA then there are various settings under "contact" to define the relation of two faces in your case an adhesive material produce bonding...

These links are regarding the FEA analysis
http://www.autodesk.mx/adsk/servlet/item?siteID=123112&id=13344407

http://www.youtube.com/watch?v=FMHSEo4YNMs

http://designandmotion.net/autodesk/mfg-pages/inventor/inventor-a-realistic-look-at-stress-analysis-contacts/

Any probs let me know

Regards