Zoom defaults to zooming into the selected object or area. If nothing is selected zooming will be into the playhead. Press Cmd-Shift-A to deselect all before you zoom in. Or depending on the version you're using, which you didn't mention, there is a keyboard shortcut that can be activated to zoom into the playhead regardless of selection.
All the best,
Class on Demand DVDs "Complete Training for FCP7," "Basic Training for FCS" and "Final Cut Express Made Easy"
Author: "Final Cut Pro 5 Editing Essentials" and "Final Cut Express 4 Editing Workshop"