Discussions has a fine grained set of user permissions which enable you to grant different capabilities to the different roles in your Sakai site. When Discussions is initially added to a worksite, the permissions may well not be setup as yet and a very limited set of functionality may be available to you. This can be remedied by turning on Discussions permissions in the worksite realm, the !site.helper realm or the !site.template project site template realm. Ask your Sakai administrator about this.
When the realms have been configured you should see the 'Permissions' link at the top of the tool area. Click on this and you will be presented with the permissions matrix for the current site. Check/clear the various permissions for the roles within your site and click 'Save' to commit your changes.