dbml file - newbie picking up a web project that needs guidance on this tech

Hi

I'm a traditional webforms asp.net 2.0 developer who uses ado.net and sql server
i've inherited a solution with code I'm trying to figure out and uses tech i've never used before

I wont go into too much detail but i have the following
Contacts.dbml file with what looks like 4 diagrams of the database tables:
Contacts
Departments
Locations
Divisions

I also have an ascx file with a linq data source that currently points to:
Contacts.ContactsDataContext
on the next screen of the wizard you select the table (and you choose from one of the 4 tables i mentioned)

I also have a grid which allows you to view and edit the data in the table

What i need to do now, is store the which Divisions are at each Location
I have manually added a 5th table in SQL Express called dbo.LocationDivisions

I need the grid to show all records in dbo.LOCATIONS
clicking EDIT will shows all the dbo.DIVISIONS as checkboxes, and you tick which divisions you wish to be assigned to that Location, and it gets saved in the new dbo.LocationDivisions table

However, i've no idea where to start with this new tech

I assume i need to focus a lot of attention on the DBML file, but have not idea what i'm doing here
Can someone help me get through this, and advise me what steps i need to take particualry on the dbml side as i'm new to this bit