WorldGuard Chest Protection is very basic protection and has a few caveats.
First, you need to place a sign as a sign-post, not on a wall.
Second, you need to write [Lock] on the first line (include the brackets), your username (typing someone else's will not work) on the second line, then up to 2 more people who can access it on the 3rd and 4th lines respectively.
Third, you need to place a chest above the sign and fill it with contents. Placing a double chest above a single sign will still work.
Errors and common mistakes
You need to make sure that you're in a region where chest-access is set to allow or the Locking is trivial and won't allow access to non-members of the region.
If you get the error message "WorldGuard's sign chest protection is disabled" you need to go to your server folder, \plugins\worldguard and open the config.yml text file. On line 93 you will see:
chest-protection enable: false
This needs to be "true" for chest protection to work.
If you get the error message "That is not a safe block that you're putting this sign on" that is because chest protection won't work on certain blocks. Such as sand, gravel, and TNT. Try something more stable.