for(int i=0; i< _dset.Tables["NameOfDataTableToFine"].Rows;i++)
                    {
                        if ((_dset.Tables["NameOfDataTableToFine"].Rows[i].ToString() == namebox.Text)
                            && (_dset.Tables["NameOfDataTableToFine"].Rows[i].ToString() == passbox.Text))
                        {
                            loginbtn.Text = "Logout";
                            label3.Text = "Active";
                        }
                        else
                        {
                            MessageBox.Show("Wrong username and/or password. Please reenter the data.", "Warning");
                        }
                    }

namespace Alocare_resurse {
	public partial class Form1 : Form {

		public Form1() { InitializeComponent(); }

		// define where you get a connection once.  You might have this code elsewhere
		private OdbcConnection GetNewConnection() {
			return new OdbcConnection("DRIVER={MySQL ODBC 3.51 Driver}"
				+";SERVER=" + "host"
				+ ";PORT=" + "port"
				+ ";DATABASE=" + "DataConnect"
				+ ";UID=" + "root"
				+ ";PWD=" + "****"
				+ ";OPTION=3";
		}
		
		private void Form1_Load(object sender, EventArgs e) {
			// I'm confused, do you alread have an adapter?
			// TODO: This line of code loads data into the 'dataSet1.users' table. You can move, or remove it, as needed.
			this.usersTableAdapter.Fill(this.dataSet1.users);
			CheckConnection();
		}
		
		private void CheckConnection() {
			OdbcConnection conn = GetNewConnection();
			try {
				conn.Open();
				MessageBox.Show("Conexiunea s-a realizat!");
			} catch (OdbcException ex) {
				MessageBox.Show("Nu s-a realizat conectarea la BD.\r\nAsigurati-va ca sunteti conectat la internet.\r\n\r\nMai multe detalii:\r\n" + ex.Message, "Database connection error", MessageBoxButtons.OK, MessageBoxIcon.Error);
			} finally {
				conn.Close();
			}
		}

		
		
		// Use a data table, we're only getting that much
		private DataTable GetUsers() {
			// we'll assume you have these two columns
			OdbcDataAdapter da = new OdbcDataAdapter("SELECT username, password FROM users", GetNewConnection());
			DataTable dt = new DataTable();
			da.Fill(dt);
			return dt;
		}

		private void loginbtn_Click(object sender, EventArgs e) {
			bool found = false;
			string enteredUserName = namebox.Text.Trim();
			string enteredPassword = namebox.Text.Trim();
			
			// no point in bothering if they're blank
			if (enteredUserName!="" && enteredPassword!="") {
				DataTable dtUsers = GetUsers();
				// loop until you find them ( or not )
				foreach (DataRow row in dtUsers.Rows) {
					if (row[0]==enteredUserName) {
						// assume we'll only have one entry for user name
						found = (row[1]==enteredPassword);
						break;
					}
				}
			}
			if (found) {
				loginbtn.Text = "Logout";
				label3.Text = "Active";
			} else{
				MessageBox.Show("Wrong username and/or password. Please reenter the data.", "Warning");
			}
		}
	}
}

for(int i=0; i< _dset.Tables["NameOfDataTableToFine"].Rows;i++) 
{

}

for(int i=0; i< _dset.Tables["NameOfDataTableToFine"].Rows.Count;i++)
{

}