To begin you will need to get the Salesforce object ID of the object you wish to link to. You can do by following the "Get the Salesforce Object ID" instructions for adding synchronous links.
Go to the JIRA issue you wish to link
Go to More Actions > Link menu, or the + symbol next to the Issue Links section heading.
The linking popup dialog should appear. This is the standard dialog for linking issues in JIRA, however there is now an extra tab on the left called Connector Link. Click this to see the following:
Choose the Salesforce connection from the drop-down connection options.