When you define an R-group in STNext and use Variables, Atoms, or Shortcuts from the R-group Definitions window, the Markush Match Level and Element Count attributes will correspond to the defaults set in the Structure Editor preferences.
When you mouse over the R-group node, no Attribute Values are highlighted in the right-hand column (and if you right-click, you see a message saying R-groups do not have attributes assigned to them).
However, if you create fragments, and then assign them to the R-group per the usual method, you may assign/edit the attributes by right-clicking the node. The R-group's current attributes are then highlighted on the right.