ServiceDesk does use and require Symantec Management Platform for licensing and Symantec Management Platform server does give a short trial license (7 days when I last looked a few versions ago) for newly installed solutions, including ServiceDesk.
For longer trial, you will have to contact a Symantec sales representative who will be able help you further.
I don't like the way our web site download functionality works but you should start by downloading a trial version of CMS or ITMS, which will in fact give you a download of SIM (Symantec Installation Manager) which in turn will allow you to download and install other Symantec products.
1. http://www.symantec.com/business/products/downloads/index.jsp
2. Infrastructure Operations > CMS/ITMS.
3. You need to log in and click the download link. What you'll get is Symantec Installation Manager installer.
Regarding the installation itself, you need to be aware that you will need at least two servers to try ServiceDesk - one for Symantec Management Platform server and the other one for ServiceDesk server.
For installation, you more than likely want to review the following documents:
- Symantec Management Platform 7.1 SP1 Installation Guide
http://www.symantec.com/docs/DOC4236
- Symantec™ ServiceDesk 7.1 SP1 Implementation Guide
http://www.symantec.com/docs/DOC3929
- Installing or Upgrading Servicedesk 7.0 MR2 to 7.1 - Using a Domain-based Service Account
http://www.symantec.com/docs/HOWTO49691