You need to download and install the program StellariumScope. Once you have done that, then you need to download and configure the ASCOM Driver and configure it for your location and setup.
The Stellarium program uses an additional program called StellariumScope to interface the PMC-Eight ASCOM driver to the Stellarium program. You need to download and install the StellariumScope program after installing the ASCOM platform software and the PMC-Eight ASCOM Driver. Once you start up Stellarium, you can start up StellariumScope and configure the ASCOM chooser and select the Explore Scientific PMC-Eight ASCOM Telescope driver. Refer to the instructions included with Stellarium an StellariumScope for the details.
It is suggested that you use the ASCOM Platform Program POTH if you are going to use multiple ASCOM clients at the same time, programs such as PHD2, and Sequence Generator Pro (SGP). Once you have the software configured correctly, you should be able to successfully connect to the PMC-Eight and navigate the sky using Stellarium.