Go to Tools>Options, and select the far left tab FILES, then the top item on the list, Support File Search Path. Click Add, then Browse, and locate the folder where your MDB file is on the network and click on it. ACE will then look in that folder for the MDB file.
To rename the original MDB file, it is in C:\Documents and Settings\[user name]\My Documents\Acade 2007\AeData\Plc
if you are using ACE 2007.
You don't have to change the ENV file for this, nor do you need to locate any other files in this folder. I have some custom PLC symbols, so I also have them in my network folder along with the MDB file. That way I don't have to copy them with each new release of ACE.