What's the error you are getting?
I see a couple of things. First...you really should name things more meaningful. textbox1 and textbox 2 is confusing. Start a naming convention with the first three characters telling you what the DataType is, then add a meaningful name. txtUserName and txtPassword are SOOO much easier to understand.
Second item, what are you trying to do here?
if (textBox2.Text == textBox3.Text && String.IsNullOrWhiteSpace(textBox1.Text) == false)
Third, you have two values in the table, but are setting three?
string insertCMD = "INSERT INTO LoginTable (User,Pass) VALUES (?,?,?)";
Fourth, what is the @ for?
string ConnStr = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=G:\\MedDesk\\Login.accdb;";
And fifth, if you're just playing around it's probably not an issue, but you really shouldn't store passwords as plain text. Look into hashing them before you save them into the file, then you can compare the hash of something a user types to what you have saved in the file. It's much more secure.