How to zoom in/out using mouse wheel? Using Yosemite, I want to use CMD + mouse wheel up/down in order to zoom in/out(CMD +/- equivalent).

How can it be achieved?
CMD + mouse wheel in most cases doesn't increase the font size, it's usually in most circumstances a 'zoom' in. Increasing the font size system wide isn't possible without setting it specifically for Applications (if you know the menu title) in keyboard shortcuts. However, you can emulate a system wide zoom in level with a double tap of the mouse (if your using magic mouse) by turning that system feature on in:

System Prefs > Mouse > Smart Zoom 

Or you can take advantage of Accessibility feature which is turned on by default by zooming the window in:

CMDOPT=

To emulate this with a mouse scroll event go to :

System Prefs > Accessibility > Zoom

You will a list of options for you customize your scroll events. 



'Use Gesture Scroll Events' will be greyed out by system default. Select this option to enable it. Choose which primary key to function from, OSx will take care of the rest (directional zoom). As you can see you can customize furthermore with picture-in-picture etc to better service your need