You can download a trial version of Workplace Designer 2.6 here. You can run it on Workplace Services Express (WSE) 2.5.x or Workplace Collaboration Services 2.6. If WSE, then you will need to install the Designer runtime code on the server. The whole installation process for Designer is about 15 minutes.
There is also a trial of WSE 2.5.1 posted in the same place. You do need the server in order to see how your code is working.
Of course, you can also download Domino trials here.
Sr. Product Manager
IBM Workplace Application Development