The tomehost interface has a search box on the left hand side, just above the treeview. This searches in two ways.
As you type, it will try to match with heading names and highlight matching ones (bold, italic and orange). However, this only matches visible headings; it won't look inside at nested headings that have not been expanded, or at the content within a heading section.
Click the search button or hit enter to submit a search. This runs a full search against all headings, and should produce more comprehensive results. It searches both headings and the content immediately under them for the entered keywords.
The search uses an 'AND' operator, and also searches content within a heading as well as the heading itself. So in this case above, the three sections contained the words 'user' and 'access' within either the heading and/or the content immediately under the heading.