Hi,
I think your ADSL probably requires PPPoE. Try this:
Go to network preferences and first note what it says in the LOCATION pop up menu (so you know how to set it back). Now in that LOCATION pop up menu, choose New location. Name it (e.g. ADSL) Then choose the ethernet adapter you are using to connect from the SHOW pop up menu. Then click on the PPPoE tab, check the "use PPPoE" checkbox and enter the information from the ADSL isp.
Once everything is set click on apply now and shut down.
You should then restart your ADSL modem, connect the MacBook with ethernet cable, and lastly power up the MacBook.
To flip back and forth between your network location, you can click on LOCATION under the Apple menu.
Hope that helps,
rjt1000