How do we link ppt to a captivate file?

I think you will find that while Adobe Captivate has the capability to import PowerPoint files, most experienced users don't use this feature. I personally would recreate content natively in Captivate rather than use the PowerPoint import feature. Any edits to your content would require you to go back to the original PowerPoint file and then reimport or update the linked file. For most these added steps aren't worth using this workflow. Also it is far from a perfect solution. If there are animations in the PowerPoint they sometimes don't get imported properly. Perhaps try removing any animations in the PowerPoint before bringing it into Captivate.