Please check if you have added enough subdivisions. Bending or twist won't work if the object does not have sufficient subdivisions. You can either add a subdivision surface modifier before the geometry node, or insert a Mesh Subdivide node within your geometry node itself.

Yes, you can use them in two different ways. 1) You can add these nodes just like a modifier (simple deform modifier) or 2) You can use them as single nodes within a larger geometry node tree. Please refer to the product document for detailed steps with necessary screenshots.

Please check if you are in the Edit mode. The nodes get imported each time you open a file. If the file is in the Edit mode while opening, the import can fail. However, in that case you can ignore this message. Go to the Object mode, and from the menu option - select Bend Geometry / Twist Geometry / Taper Geometry / Stretch Geometry as needed. The system will import the nodes again.