Try this:
//using System.Security.Cryptography.X509Certificates;
public static X509Certificate2 selectCert(StoreName store, StoreLocation location, string windowTitle, string windowMsg)
{
X509Certificate2 certSelected = null;
X509Store x509Store = new X509Store(store, location);
x509Store.Open(OpenFlags.ReadOnly);
X509Certificate2Collection col = x509Store.Certificates;
X509Certificate2Collection sel = X509Certificate2UI.SelectFromCollection(col, windowTitle, windowMsg, X509SelectionFlag.SingleSelection);
if (sel.Count > 0)
{
X509Certificate2Enumerator en = sel.GetEnumerator();
en.MoveNext();
certSelected = en.Current;
}
x509Store.Close();
return certSelected;
}