After copying tnsnames.ora to the ORAHOME folder under \Network\ADMIN it is possible to create e.g. a System DSN which is accessible by Admin.exe
Now Admin.exe can connect to this database and it is possible to create the administrative DigaSystem tables. But of course please be sure that the SQLAdmin section for your Oracle database is present before executing any command in Admin.exe. The driver name must correspondent with the section below Global Settings \ SQLAdmin.
Please restart Admin.exe after adding new sections for SQLAdmin!

Now it is possible to create the administrative tables. Please note that you have to modify the setup before restarting Admin.exe. You need to create the main section for the new database and add some parameters.
Global Settings \ Digas \ Database \ ... (replace ... with the technical name of your database):
Owner = DAVID UppercaseFunction = LOWER
Now it is possible to access the content of the new database after restarting Admin.exe

Although to date, DAVID Systems has not completed testing of Oracle for 64bit, Michael Burkhardt has suggested a tip for enabling the functionality of OraClient11g_home1;
Oracle offers a new version 11 of their ODBC drivers for 64 Bit that will be installed as Oracle in OraClient11g_home1.  When coming from driver version 10 and using FileDSN it is necessary to modify the FileDSN and replace the driver name inside the DSN file. Additional in Admin tool a new subkey below Global Settings \ SQLAdmin must be created using the same name the ODBC driver is using to identify itself. You can copy the parameters of version 10 to this new section