You cannot put a method call into the LINQ query that the provider cannot translate into SQL. So, you cannot use .ToString().
The quick'n'dirty solution is to retrieve the objects, convert it to a list so the query executes against the database and then create your view objects from the list, putting the .ToString() call in that code instead.
void BindGrid()
{
using (QTEL_Entities qtl = new QTEL_Entities())
{
var dataItems = qtl.STATIONs.ToList();
var rslt = from item in dataItems
select new
{
id=q.ID_STATION,
lib=q.LIBELE_STATION,
typ=q.TYPE_STATION,
dsm=q.DATE_MS.ToString("yyyy-MM-dd")
};
gridSample.DataSource = rslt.ToList();
gridSample.DataBind();
}
}