In the Navigator when first opening PM+ (Build 4910), there’s no access to subfolders. Double-clicking will not open them.
However, collapsing the main folder and then reopening it provides access to the subfolders (triangles shown).
See the screen grabs.
Images on an external Seagate SSD.
My system:
macOS Catalina version 10.15.6
iMac (Retina 5K, 27-inch, 2017)
Processor 4.2 GHz Quad-Core Intel Core i7
Memory 32 GB 2400 MHz DDR4
Graphics Radeon Pro 580 8 GB