You don't so much "install" a dropdown as apply an option name/value to a product.

First, you have created the option name - did you select for it to be a dropdown type?

Once you have the option name and some values associated with it, go into Attributes Controller, navigate to the product you want, and add attributes to it specifying the option name, option value, price etc. (Until you have at least two values for an attribute, the dropdown will show as a radiobutton.)

You might also check the FAQ/tutorials for "attributes" - there is a lot of good info there.