Thea Render v1.5 is compatible with NVIDIA CUDA 6.5 and GPUs that have compute capability 2.0 upwards. To determine what type of compute capability your GPU has, we highly recommend you visit the Wiki page for CUDA available here: https://en.wikipedia.org/wiki/CUDA
This Wiki entry provides a detailed analysis as well as a table of all available GPU cores along with corresponding compute capability. If your graphics card meets the minima requirement for compute capability, has support for CUDA 6.5 but is still not visible in the Device Panel then it is imperative that you perform an update of your graphics driver.
Please note that in relation to NVIDIA Pascal generation graphics adapters we are currently providing a preliminary update supporting GTX 1060/1070/1080 and Titan X. This update is available in the v1.5 download thread and also available in the main public download section. You are required to manually replace the Thea Presto plugin dynamic libraries by expanding the available zip bundle and replacing the files in C:\Program Files\Thea Render\Presto\Plugins.