Q: Problem with zoom tool in the slideshow

Which version of iPhoto are you using?  Or are you using the new Photos.app?

I hope that there is some trick to zoom

Are you referring to the Ken Burns theme in iPhoto? There's a similar theme in Photos. When you activate that theme click on the button highlighted in this screenshot to bring up the zoom tools:

zoom tool can only be used in Ken Burns?

Yes.

In older versions of iPhoto could be used in all subjects,

Not in slideshows.  In the other viewing window you could use the sidebar to enlarge the image and that can be done with Photos.

Are you referring to the scaling of the slideshow to fit the screen?  If so then yes it can be done like in iPhoto:

iPhoto:

Photos:

If any of these is not what you're referring to you'll have to give is a more detailed description of what it is.