when you zoom closer, the camera is moved toward the camera target point. The target point initially is in a center of scene (0,0,0 point), but you can move camera target hold the middle mouse button (wheel) pushed (or both left and right buttons pushed). having a camera target focused on another point might help you zoom closer to some details.
another important thing is a camera's front clipping plane, the plane that clips geometry "too close" and behind the camera. When you zoom (and camera moves toward), clip plane reaches the model you you see it sliced model. There is one trick to prevent this behavior: the "filed of view". When you change field of view (camera lens), camera cam move far far away, but can still show you a model on high zoom. Hold down Shift key, right mouse button and move mouse up/down to change field of view. You will see the "prespective" changing to "isometry". The more "isometry" the view, the further camera was moved away. The further away - the higher zoom you can reach before "clipping" will occur.
Also, regarding your explanation - I guess you were talking about ZModeler v1.x ? It requires DirectX 7 and might not work correctly on modern OS like Win 8, 7 or vista.