You should be able to add this query through your Admin CP or by using a program called MyPHPAdmin.
In the Control Panel goto "Import & Maintenance" (usually located at the bottom of the control panel), then click on "Execute SQL Query". In the "Manual Query" box you can type the SQL query
Code:
ALTER TABLE user ADD pmsound varchar(7) NOT NULL
then click on "continue" to apply it.
Hope that helps.