You must have a Paragraph Text type layer in your file. After you log in, click "Add new variable". Then click "Select the correspondent layer" in the drop-down menu. Fill in the requirements concerning this variable:
Whether this text will have a color variable
Limited number of characters or "Smart Resize"
Then click "Save".
To add a text variable, you must create a text layer in Paragraph Text style.
Go to the Variables tab, select the text layer from the drop down menu and set the name.
Next, check the following:
Is variable is mandatory
If it will get a color input
Set the maximum character limit.
Note: To add color to a text or a shape variable, you need to enable a text or a shape layer by going to "Layer Style" > "Color Overlay".
Then just click on "Add Variable", and it will appear in your list of registered variables.