There is a hint on plugin documentation page:
The plugin requires the StarTeam SDK, StarTeam extensions and jsafe libraries in Hudson's classpath. These files are included in the freely downloadable StarTeam SDK package from Borland (Micro Focus) , and may be installed by e.g. dropping them in Hudson's WEB-INF/lib directory.
Copy the folder "C:\Program Files (x86)\Borland\StarTeam SDK 10.4\Lib" to "C:\Jenkins\plugins\starteam\WEB-INF" (so there would be a Lib folder under WEB-INF), and restart Jenkins.
If you get "Can't load IA 32-bit .dll on a AMD 64-bit platform" error:
Uninstall JRE (Java Runtime Environment) for Windows x64, install 32-bit version of JRE.