how to pull up downloaded projects on Autodesk Inventor?

Weird!!! Is it a future version problem??
Or perhaps try changing your Internet browser (chrome is OK)

when you download inventor files please make sure you have the right type and have all the files needed to open it. for instance just a project folder does you no good. renders do you no good. step files will only show you a model but you cannot change or edit anything. also if you download an assembly you can only open it if you also have the parts in the right folder cause the assembly will refer to those files.
hope it helps you

If standard parts from the content centre have been used but not copied-in properly, they will cause unresolved links. It is possible to open the assembly with those links still unresolved and repair the link by right clicking the standard component in the model browser. HTH.