With Notes you can access or create new notes in either the program Gnote or Tomboy. Notes will work identically with either application—you can configure which one to use in the plugin's information pane in Kupfer Preferences.
Creating a new note
Activate Kupfer and type . to enter text mode, then type in a note title
Type Tab and select the action Create Note and press Return.
Finding a note by title
Simply search for the note title in Kupfer
Finding a note by content
Activate Kupfer and type . to enter text mode, then type in a search query
Type Tab and select the action Get Note Search Results... and press Return.
Power User Tips
You can use the Selected Text object and create a new note with the content of the selection.