Check the text file you received from PTC for your license file. Does it contain a line (or lines) for Creo Schematics Lite?
You may need to contact Order and License Management. The Engineer III license should come with PCX (Piping and Cabling Extension). That grants you access to Creo Schematics Lite (but not the regular full license). You should be able to do most of what you need with the Lite license.