If I understand the question correctly, you're trying to build an IN list in the WHERE clause. If this is the case, perhaps something like
OleDbCommand lct = new OleDbCommand("UPDATE [Or] SET [Loc] IN ", MAcon);
string inContent = "";
for (int i = 0; i < CustLB.Items.Count; i++) {
inContent += $"@loc{i},";
lct.Parameters.AddWithValue($"@loc{i}", CustLB.Items[i].ToString());
}
inContent = inContent.TrimEnd(',');
lct.CommandText += $"({inContent})";
Also as already pointed out, try to avoid reserved words in names.