My boss bought a Metrohm autotitrator for TAN/TBN analysis. But nobody knows how to operate it. The manuals didn't help much. Can anyone give me some basic information/method on how to use it to do the testings mentioned?

You need to purchase the ASTM D-2896 method or similar.

The metrohm titrator is a highly sophisticated and very accurate autotitrator working on the principle potentiometric titration.The EMF(electro motive force)has a vital role to play in this instrument.So be cautious while installing and operating this machine.It would be better for you to call the metrohm service engineers to your location and let them to install it.Then you can devolep the methods for TBN/TAN as per the operating manual(ASTM) by using the key board.If you are provided with more than one autoexchange unit,you can devolep more methods.ie TBN,TAN,WATER CONTENT etc..as per the ASTM methods.The metrohm operating manual is made as per the ASTM methods.So just follow the procedure in the operating manual step by step to develop the methods.
If you want more information,you can contact me through the address given below.
Thanks.

nnkrishnakumar_menon@rediffmail.com

Methrohm provides support in it's Applications part of their internet site - and your local agent can also provide their Application Notes. In any case these titrations - espcially TAN, are not trivial - so you'll need patience.