WORKAROUND 1:
Alter user's default schema with the following scripts.
ALTER USER <user name> WITH DEFAULT_SCHEMA=[dbo]
or
ALTER USER <user name> WITH DEFAULT_SCHEMA=[guest]
WORKAROUND 2:
If the user login is added as an NT group to the SQL Server and not a particular windows user, the default schema returns null. When connecting to SQL Server using Windows Group, you cannot set a default schema for the group.
In this case you may just have to adjust your login and connect as Windows Auth.
1. Connect to the SQL Server with a login who has permission to create a user, and then use the following script to create a user for your login(pls replace the domainname\username with your NT login).
USE [Brevetto]
GO
CREATE USER [domainname\username] FOR LOGIN [domainname\username] WITH DEFAULT_SCHEMA=[dbo]
GO
After the user above is created, connect to database with Windows Auth and you should be able to use SQL Optimizer to tune his SQL.
or
Have the DBA directly add your NT user as a login.