I couldn't find a quick example when I was looking for one, so here's mine.

First, create your stored procedure with an output parameter:

CREATE PROCEDURE [dbo].[MySproc]
(@MyParameter varchar(50) OUTPUT)

Then, add an output parameter to your database command:

Dim _db As Database = DatabaseFactory.CreateDatabase
Dim _cmd As DbCommand = _db.GetStoredProcCommand("MySproc")
_db.AddOutParameter(_cmd, "@MyParameter", DbType.Int32, 4)

Then, execute your stored procedure, and assign the parameter value to a variable (remember to DIM the variable)

_db.ExecuteNonQuery(_cmd)
_result = _db.GetParameterValue(_cmd, "@MyParameter")