@nitnatnoe said:
I was wondering if I can download the NDI Watcher and Screen Capture actor in the Isadora Demo Version
Yes, the demo version supports everything the full version supports (except for saving). As @CitizenJoe has said, and the dialogs outline, you can download the required files from the Add-Ons page, and install them to the 'Isadora Actors Plugins' folder that can be opened directly from the menu option under Help in Isadora.