Either use an existing variable or create a new variable in Google Tag Manager. To create a new variable, switch to the Variables area on the left in the navigation and click on the NEW button at the bottom of the Custom Variables area.
Name the new variable in accordance with the use within the tag (for example, optinemail), and typically select data-layer variable as the variable type to use with an existing data-layer variable (for example, optinemail).
For further information about using variables in Google Tag Manager, see here: http://support.google.com/tagmanager/answer/6106899? hl = en