3dcart is discontinuing their SOAP API on 06/06/2016 and this will require you to use their REST API to sync properly with ShipStation.
First, login to 3dcart and click Modules on the lefthand sidebar.
Now, click the + icon on the Rest API row and click the Change Settings button that appears.
On the REST API page that opens, click the + Add button in the upper righthand corner:
Enter 9ff5a12b310ec4ea9d22666ecaf249d8 into the "Public Key:" field that appears and click + Add.
On the popup that appears, select + Authorize.
A new ShipStation popup with the 3dcart modal will now appear. (Make sure no popup blockers are enabled.)
Select your Product Weight Units from the dropdown menu and click Connect.
Your 3dcart store is now updated! ShipStation will open in a new tab where you can further customize your store's information!