Hi Gray,
You can achieve this using conditional formatting. Follow the below steps.
1. Add a text box to the form.
2. Go to Text box properties.
3. Go to Display tab -> conditional formatting.
4. Click Add button.
5. Select the drop down list field in the first drop down, is not equal to in second drop down, enter the value (for which option you want to display text box) in third drop down.
6. Check ‘Hide this control'.
7. Click OK until all the windows gets close.