If you prefer the "vanilla" ImageJ experience, you must manually install the plugin and its dependencies: Go to the Fiji Wiki page for TWS.
Are you planning to use TWS for or are you working with 3D stacks and volumetric data? trainable weka segmentation plugin download
Use the ROI tools (line or point) to select pixels belonging to each class. Click Add to Class 1 or Add to Class 2 . If you prefer the "vanilla" ImageJ experience, you