If you already have 32-bit Office installed then you're pretty much stuck with the 32-bit version of the Access Database Engine (a.k.a. "ACE"). As you have found, there is supposedly a way to force 64-bit ACE onto a machine that already has 32-bit Office components on it, but personally I wouldn't "go there".
So, you need to configure your C# project to run as 32-bit. You can do that by choosing Platform: x86
on the Build tab of the project's Properties:
(That screenshot was taken on a 32-bit virtual machine, so the default configuration was already 32-bit. The default setting on 64-bit machines should be "Any CPU".)
To verify the environment in which the process is running you can use
String.Format("I am running as {0}-bit.", IntPtr.Size * 8)
For example, in a Windows Forms application you could use
MessageBox.Show(String.Format("I am running as {0}-bit.", IntPtr.Size * 8));
That should display
I am running as 32-bit.