When you register for the 120 trial version you should see a Download Foundation Server button in My Downloads widget (under the Tutorials) at https://developer.thingworx.com/dashboard . This download include the Thingworx.jar and does not require access to the PTC Software download page.
Hope this helps
Thank you very much, Christophe.
I was able to download the Java SDK and demonstrate connectivity to my local ThingWorx Composer from my local Eclipse IDE and from a Raspberry Pi.
Is it possible to to use the 30-day trial (simultaneously with the 120-day developer trial) to evaluate the PTC-hosted ThingWorx Composer?
William - you can utilize both the 30-day hosted server as well as the 120-day downloadable trial. They are independent of one another. The intent of the hosted server is to allow developers to explore and learn the technology while the 120-day downloadable version is intended to provide more time and control while creating your own applications or proof of concepts.