Click on your user name on the right side of your screen and click the "my store" link.
Click on the tab marked "products", find the product you'd like to add an option to, and then click "edit."
Now look for "options" on the left side, click, and then select "+ New Option."
Here, you can put in all of the details of your option. To add more than one option, just click the bottom right button marked "new option." Here is an example of a filled out form:
And here is what buyers will see on my page: