I was able to find the PowerPoint Conversion assembly in the GAC - but for some reason it is not present in the SharePoint 2013 ISAPI directory. It would have been nice if Microsoft included it in the Sharepoint 15 hive or had the proper registry key so it would appear in the Add Reference dialog - but we can easily extract it from the GAC or copy the GAC path when adding a reference in Visual Studio.
Microsoft.Office.Server.PowerPoint.dll Location
\Windows\Microsoft.NET\assembly\GAC_MSIL\Microsoft.Office.Server.PowerPoint\v4.0_15.0.0.0__71e9bce111e9429c\Microsoft.Office.Server.PowerPoint.dll