What processing level did you download from EarthExplorer? "Landsat Collection 1 Level 1" is what we recommend. Also, what version of Geomatica are you using (in Focus, Help-->About).
We recommend opening Landsat imagery using the "keyfile", which is named as follows for all Landsat packaged data from USGS:
Drag/drop this file into Focus (or open in any of the Geomatica tools) and you will be prompted to open either the PAN band, the MS bands, or the Thermal band. To get all bands to load correctly (for the multi-spectral), choose "-MS"
You can review how to open Landsat in the help:
As for the red X, that's because the file is not imported to PIX. Landsat imagery contains multiple raster channels (Pan, MS, Thermal, Quality). Once you import to PIX you will be able to add channels, or write to the file. The temporary import/view is not writable, thus the red x.
To create a full dataset with all bands at the same resolution you will need to create pansharpened data (Algorithm Librarian-->PANSHARP). This will take all the MS bands and resample them to 15m and retain the spectral content. A better algorithm to preserve radiometry while pansharpening (for scientific analysis) is MRAFUSION. It works in a similar way, but has been designed to preserve radiometric content in original imagery.
If you follow these guidelines you should be fine. If you search "Geomatica Atmospheric Correction" on YouTube you will see many examples of tutorial videos for Landsat 8 Atmospheric correction. PCI's last video on this topic was done 3 years ago, and since then our technology has changed again quite a bit, but the steps are similar. See the video here: https://youtu.be/5ATyM-bbyc8.
Kevin (PCI Geomatics)