SharePoint/InfoPath: Get item Attachment URL for link

In InfoPath add a hyperlink (under the INSERT tab)

Under link to click Data Source.
Click the data source icon
 – Click Show advanced view at the bottom
   – Click the “+” next to dataFields
     – Click the “+” next to my:SharePointListItem_RW
       –  Click the “+” next to Attachments
         – Click on :attachmentURL
Click OK

Beside Display choose whatever text you would like it to display.

Click OK