There are 2 ways to add Product Options. 

1. Global Product Options. (that are added in this area: YourDomain.com > Modules > Ecommerce: Products (mX) > Product Options ) These options can apply to multiple products.  For example: if you have T-shirts and sell different sizes you can setup sizes that can apply to multiple items.  Here is a quick video on how to do this.

2. Per Item Product Options.  These options are setup within the product.  They apply only to the product that you are adding the options to.

Here are some videos on the different product options.

Here is a brief description for each option type:

Enum - is used for creating product options where the Buyer can select (one or more) options from a list of options. Each product can have multiple Enum options if needed. Enum options can have weight and price modifiers -- in case the one or more of the available options needs to increase or decrease the product's weight or price.

Tree - is used when options in the list need to be categorized. For example: Select Make -> Select Model. There the list of models would be dependant upon the select Make.

Custom Text - is used when you need the Buyer to have the option to supply text with the product. For example: a Gift Message.

Custom Price - is used when you need the Buyer to specify the price they will pay. This is most commonly used for a selling a Gift Certificate, or if you use the website to collect online payments.

Custom File - allows Buyers to send a file with their product/order. This is common with Customized products (engaved products, for example).

Custom Number - is similar to Custom Text - however it collects numbers only.

Custom Date - allows Buyers to provide a date with the product/order. For example: selecting a delivery a date.

Custom Weight - allows buyers to supply a weight for the purchase. Keep in mind, if your shipping rates are calculated by weight, this will affect shipping costs.

Global Product options that apply to multiple products are setup here:

Here is a list of Global Product option types:

Enum is one of the more common Product Options.  It has many uses.  For example you would use Enum for the example of shirt sizes.  You can set up as many in the list as you would like, and can choose if the option increases or decreases the price of the product.  You can also choose if the option will increase or decrease the weight of the product.  See example below:

Tree - is used when options in the list need to be categorized. For example: Select Make -> Select Model. There the list of models would be dependant upon the select Make.

Custom Text - is used when you need the Buyer to have the option to supply text with the product. For example: a Gift Message.

Custom Price - is used when you need the Buyer to specify the price they will pay. This is most commonly used for a selling a Gift Certificate, or if you use the website to collect online payments.

Custom File - allows Buyers to send a file with their product/order. This is common with Customized products (engaved products, for example).

Custom Number - is similar to Custom Text - however it collects numbers only.

Custom Date - allows Buyers to provide a date with the product/order. For example: selecting a delivery a date.

Custom Weight - allows buyers to supply a weight for the purchase. Keep in mind, if your shipping rates are calculated by weight, this will affect shipping costs.