i odn't know if backup exec has a plugin for sql express databases or maybe it only works for proper sql database (my backup software only recognises proper sql and not sql express)
i have configured a manual backup on my sql express server using this link
How to Automate SQL Server Express Backups - Tip of the Week -
i basically run separate scripts for each database on the sql express server and then backup the server (including these sql backup files) and once per week i delete the sql backups again using a script.
not sure if this is the answer you are looking for but it works for me