@martinfromaeschi
I signed up for this forum after many frustrating hours fiddling with itunes but i feel i might have found an answer. This might sound overly simple or redudant in a way but hear me out.
I just recently got a Windows 7 computer and when i transfered my playlist I was having the same problem where I couldn't search comments field. But, on my previous comp, a Win Vista, it worked fine.
The suggestion of activating or clicking on "comments" field on the list is correct HOWEVER, you have to do so in the TEXT LIST VIEW [first viewing option from left,] that is left of the search box ((i'm saying this assuming Mac itunes have similar if not same UI as the windows one))
Since when i had my vista i used to view the music on TEXT LIST VIEW [first viewing option] but i migrated over to ALBUM ART LIST VIEW [2nd viewing option] and obviously comments was activated on the list view long before so i didn't notice any issues with searching comments.
But by default that's not the case. ***You have to enable "comments" field from the TEXT LIST VIEW*** Even if you don't have 'comments" visible in other viewing options when you search, as long as "comments" is visible from the TEXT LIST VIEW, you can search "comments" field.
I don't know how useful this info is but i hope your case is the same as mine.