If you lost key or dont have the backup of the key then I would suggset you do these things.
1)Take the backup of Export folder in DAC which you copied from BI APPS installation directories.
2) As it is a new install drop the DAC schema in the DB and recreate one.
3) Reinstall the DAC server and client.While installing there is an option to mention the secuity key by yourself instead of generating random key.
Hope this helps ,if so mark correct of helpful,.