The subdirectories names should avoid blanks or underscores. If using ImageJ 1.19 or earlier, the files in the pcp subdirectory must be moved to the main plugins directory.
Pages are selectable using a horizontal row of tabs at the top edge of the notebook.
Each tab gets its name from the subdirectory containing the plugins collection. Right-click in the narrow space under the tab row pops up a menu for page selection. The tabbed panel supports more than one level of directory branching. However, the control panel will display one row of tabs, regardless of the directory tree depth.