Revit 2014 and Earlier
In Revit 2014 and earlier, you cannot import or link an IFC file directly. You have to open the IFC file in Revit, save it as a Revit project, then link the Revit project.
To do this, select the Revit Button, which is the big “R” in the top-left. Hover your mouse over “Open”, but don’t select it. Then select the “IFC” command.
The keyboard shortcut sequence for this is ALT+F Y I.
Select the IFC file you want to import.
After the IFC file is imported, save it as a Revit file. You can now use this Revit file to check for collisions with other Revit files.
Revit 2015 and Later
In Revit 2015 and later, you can link an IFC file without having to convert it.
Use the Insert->Link IFC command to link the IFC file.