There are three reasons why you may not be able to successfully import from Smartsheet:
A. You don't have Office Timeline version 3.1.05 or newer.
B. You are behind a corporate firewall that's preventing access to Smartsheet.
C. You need to run PowerPoint with additional permissions.
Try the following solutions (A, B, C) in this order:
A. Make sure you have Office Timeline version 3.08.01 or newer
To check what version you have installed, open PowerPoint and, on the Office Timeline ribbon, click Help > About.
If you don't have version 3.01.05 or greater, go to https://www.officetimeline.com/updates and install the latest build.
Try importing from Smartsheet.
B. Configure Office Timeline to use your Web Proxy
In PowerPoint, click Settings on the Office Timeline ribbon.
Click on Proxy and then choose Custom under Web Proxy Settings to specify your web proxy settings. You will need to contact your IT Helpdesk to obtain this information.
Make sure you test the settings once you've entered the URL, Port, and Username / Password (if that's required to access the web proxy).
Click OK and try importing from Smartsheet again.
C. Run PowerPoint with additional permissions
Often times, corporate security policies prevent applications such as PowerPoint from accessing the network. Running PowerPoint with elevated privileges will allow PowerPoint and Office Timeline to connect to Smartsheet. Here's how to run PowerPoint with elevated privileges:
Follow the steps in this article: http://support.officetimeline.com/hc/en-us/articles/205345867.
Open PowerPoint as you normally do and test Smartsheet.
Here's a video that will help you get started with SmartSheet Import: