Can’t modify my Canvas size on FIREWORKS CS3

Hi everybody i can’t modify my Canvas size on Fire Works CS3
i’m going to: Modify Menu > Canvas > anvas Size then i want to put the height to 30 pxs , to fit my Navigation bar Height , and put the ancer point on the top, (to take off the white space)
but when i click OK , i have this error message
Translation to English of the error text : ” The operation can’t be done , bad selection for that kind of operation “
any help would be so apreciated
thx in advence..
Answer #1
no answer ? ???????
Answer #2
Are they slices?