How to add a rule condition from select-list in a webform
You can use a technique similar to what is described in my answer to "How to do a data comparison to submitted webform data when using the Rules module?". For short:


Add an extra Rules Action to Add a variable, with a value assigned to it equal to your [data:newsletter-value-raw] (assuming newsletter is the form component containing your select-option) and with a data type which equals the data type of your Webform component. In your subsequent Rules Actions you can refer to the value of this variable you just added (that's the clue!).
Use the Conditional Rules module to perform a data comparison. In this case to check the value of your select-option. If so, and only then, perform your Rules Action to add the email address as a subscriber.


PS: Instead of using the Conditional Rules module, you could also rework the Rules Action to performing Rules Components (in which you add the appropriate Rules Conditions), so that you don't need this extra module. For more details on that, refer to the answer to "What&#39;s the alternative to using the Conditional Rules module?".