In the main menu go to Processing data and then click on the Edit button.

Select the DataSet in which you want to enter new variables and press in Create new variable.

The following step is to insert a name for the new variable and assign a value type. Value types can be: CATEGORICAL, NUMERIC or STRING.
(e.g. Phenotypic variables have, frequently, CATEGORICAL values, such as 'control' and 'disease'. This kind of information is used afterward in differential expression analysis or predictors.)

Click on the magic wand and insert the first value for this variable. You can also add a brief description (optional). For the second value or category you have to click the magic wand again and the same for the rest of values:

Click to open th edition table.

Assign the correct value to each sample. The values must be the same that the ones typed in step 4. Click on Submit button to confirm the edition.