Finding the Header Block in the Proteus symbol library
Eeeeeeeeeeeasy. It only took me four hours to find it originally!

Within a design sheet select "2D Graphics Symbol Mode" - the icon is an 'S' in a green box, next to the line/rectangle/circle graphics. Either click the P at the top of the (currently empty) list box or double click in the box itself. This opens the Pick Symbols window. HEADER is found in the SYSTEM library, between DPST_1 and INVERTER (not that I'm questioning your spelling ability). Double click header then ESC or close the Pick Symbols window.

Place a Header somewhere on the sheet. You can't edit the default header so you have make your own. You can, in fact, make your own from scratch without doing any of what I said above, but it's a bit easier to start somewhere. Right click on the header and then DECOMPOSE. This will break the header into its constituent parts. Create, destroy, add rude words, do what you like (you've seen the table in the Help files following your quoted text for the '@' keywords). When you're happy with your new header, highlight the whole thing, right click then select MAKE SYMBOL. Give it a name and you're finished.

To get it back you find it in the USERSYM library, not the SYSTEM library.