Are you trying to have a parameter available to the users so that they can select another user but it defaults to themselves?  Or are you just trying to limit the results for another parameter list to those relevant only to the current user (ie they can't select another user and thereby change the subsequent parameter values)?  If it's the latter, then you could use a dynamic query string in your dataset e.g.    instead of just Select * from Mytable where user_id =  @parm1   

you would have ="select * from mytable where user_id = '" & user!userID.Value & "'" 

Note am assuming an MSSQL datasource hence the single quotes around the user!userID.Value