When you have CS3 you can insert a HTML snippet (main menu > Insert > HTML
Snippet). The admin has to give you the proper rights.
Set rights:
1. go to main menu Edit>Administer websites>Users and Roles>Edit role
settings>Editing
2. check the option to insert HTML Snippets

If you have an older version you have to insert the code using an external
application (Main menu> File > Actions > Edit page source in external
application). In this case the admin also needs to give you proper rights.
Set rights:
1. go to main menu Edit>Administer websites>Users and Roles>Edit role
settings>Editing
2. check the option to enable editing in external appllication