How to activate the upload file field button?

In your Form Builder, select Basic Fields then click and drag File Upload field to your form. Here's a screenshot:

Please check these related guides about the File Upload field:

https://www.jotform.com/help/33-Changing-the-upload-size-limit
https://www.jotform.com/help/29-Which-file-types-are-allowed-for-the-upload-field

Hope that helps. Thank you!