I have contacted Ken LaRue over at the Autodesk Combustion forum (hosted by the CreativeCow too), and he’s used the new IELs in C4 with no problems, so you must be doing something wrong. On the library control panel just click load library and select any of the IEL files and click OK.
Combustion was able to load IEL files at one point (at least in C2, not sure about C3) — they may have removed that ability so it only can load ELC files now. Can anybody confirm?
I think you downloaded the wrong file. If you downloaded IL3 files, that’s wrong. You need to download the IEL files. IL3 is the pI3 format that Combustion can’t use. IEL is the pI2 format that Combustion can use.
The stage size must fit in your stage window at 100% zoom. If your desktop resolution isn’t high enough, you’ll have to use pIRender. pIRender is included with pI3, but the OS X version of pIRender hasn’t been released yet.