How do I zoom in with my bow?

The Block button is the Left Hand action, which is, by default, the M2 (right) mouse button. You can check what the Left Hand action is mapped to in the Controls section of the main menu (Esc).

The assigned button/key should zoom in the bow when pressed once you've started to draw the arrow, as illustrated in the screenshot below:

If you haven't drawn an arrow yet, pressing the Block button will cause you to thrust your bow out in front of you in a bashing motion. If you haven't yet unlocked the Eagle Eye skill, pressing the Block button with an arrow drawn will simply not do anything.

Searching around suggests that a handful or so of people have had a problem where despite having the Eagle Eye perk, pressing the Block button with an arrow drawn doesn't have the expected outcome. This may be because the perk hasn't been applied correctly for some reason. If this happens to you, and you're willing to resort to console commands, you could try removing and reapplying the perk by typing the following:

~
player.removeperk 00058f61
player.addperk 00058f61

Failing this, you can also try resetting your bindings to the default values by pressing T on the Controls screen, although I'm not sure if this would have any impact.