ADS40 data usually comes in TIFF format with several ancillary datasets to allow you to orthorectify and extract DEMs from the data.  Even without the ancillary data, a TIFF file can be opened.  Are your data in TIFF format?  If so, are you using File->Open External File->ADS40?

Thanks for your reply. The jobs that we fly are quite large so 1 flight line consists of a number of tiffs. We use the socet .sup to view them as one flight. Ultimately I would like to use L1 RGB Stereo pairs to trial the DEM extraction module. Can ENVI open L1imagery or only L0 imagery?

ENVI should be able to open both since they are TIFF files but they won't be opened as a single flight line, I don't think, because ENVI doesn't read the sup file to 'mosaic' them.  If the flight information that would be used to build RPCs pertains to the entire flight line of several tiff files, I would think that the tiff files would need to be mosaicked first, then build the RPCs for the entire scene using the flight information.  You will need to build RPCs for the stereo pair (a set for each image) before using the DEM Extraction tool.