Open the file /includes/page_header.php
If you want to put [admin] you should find the following lines:
if ( $row['user_level'] == ADMIN ) { $row['username'] = '[admin] <b>' . $row['username'] . '</b>'; $style_color = 'style="color:#' . $theme['fontcolor3'] . '"'; }

replace them with:

if ( $row['user_level'] == ADMIN ) { $row['username'] = '[admin] <b>' . $row['username'] . '</b>'; $style_color = 'style="color:#' . $theme['fontcolor3'] . '"'; }

For [mod] find the following lines:

else if ( $row['user_level'] == MOD ) { $row['username'] = '<b>' . $row['username'] . '</b>';

replace them with:

else if ( $row['user_level'] == MOD ) { $row['username'] = '[mod] <b>' . $row['username'] . '</b>';

If you need to specify more ranks you should find the line:

else if ( $row['user_level'] == THE RANK ) $row['username'] = '<b>' . $row['username'] . '</b>';

just before the b tag you need to put the desired rank(for example [VIP]) and put one space so it won't stick to the nickname.

I hope this was useful for you.