You can get a trial version of Rhino3d and Grasshopper from the Rhino3d website. Run Rhinoceros and select the template with Meters:
In the Command line, please enter "Package Manager":
A new window appears; select IDEAStatiCa and click Install. The plugin will be downloaded and installed. To become active, a restart of Rhinoceros is necessary.
To start Grasshopper with the IDEA StatiCa plugin, select Tools / Grasshopper in the Rhihoceros tab.
As you can see, the plugin is already incorporated.
Learn about the components and commands in the IDEA StatiCa plugin for Grasshopper in the article Description of Grasshopper plugin components.