It’s possible to zoom in and out of the map in a number of ways. Firstly there is the In and Out buttons, but it’s also possible to select “Set Scale” from the “View” menu, or press CTRL + S, or double click on the scale displayed in the bottom right hand corner. This will display a form from which you can type in the new scale.
Also note that when you are zooming in using the +in button, if you hold down the left mouse button, you can drag out a box that defines your new view. You can also use the Page Up / Page Down keys.