You need:
l2encdec.exe
L2dat_full.exe
Use l2encdec to decode the dat:
l2encdec -s archive.dat
it generates dec-archive.dat, change back the name to archive.dat
And then you use L2dat_full to make it readable!
There's also a pack with most of the decrypted files as .txt and the Encoder. Look in http://hosted.filefront.com/ForeverBluE for L2clientdat_en.rar
That will help you with most of the DATs...
For the rest you'll have to use L2 Asm & Disasm (I can't get the hang of it yet), from http://dstuff.l2wh.com/
That's all i know :oops: