Papyrus Designer is an extension of the Papyrus modeler. In order to install it, there are three different methods (CAVEAT: currently only the nightly update site are working properly with a Papyrus 2020-12 or newer, for 2020-06 the associated nightly update site is recommended as it contains several bug fixes and enhancements that are not yet in the release)
2. Install Papyrus Software designer via the market place].
3. Open the standard Eclipse installation dialog (Help->Install New Software). Within, add either the release update site of SW designer: https://download.eclipse.org/modeling/mdt/papyrus/components/designer/ or a nightly build , .
Papyrus vs. Papyrus SW designer versions
Papyrus SW Designer is an extension of Papyrus, but it is not part of the release train and released independently of Papyrus itself. Therefore, the version numbers differ.
The current 2.0 release of SW Designer is compatible with Papyrus 5.0, but the nightly builds have been made compatible with Papyrus 6.0 (while maintaining compatibility with Papyrus 5.0). A 2.1 release of Papyrus SW designer that is compatible with Papyrus 5.0/6.0 is planned for January 2022.
A good starting point for Papyrus Designer are example models containing comments. These examples are available via File->New->Example, category "Papyrus" examples.