If you are serious about Microsoft Access User-level Security in earlier versions (.mdb files) of Access Databases and would like to continue using them in Access2007, with the security settings intact, then you should not convert them into Access2007 (.accdb). Access2007 Security concepts are totally different and once the .mdb files are upgraded into .accdb (Access2007) format the User-level Security settings are lost and you will no longer be able to implement them in .accdb databases.
If you have already converted .mdb database into .accdb format and lost all user-level security settings then you are not totally lost with the database. If you have a backup of the earlier version database then you are lucky. All you have to do is to restore the database from backup and use it in Access2007 with User-level security intact. If you lost the backups too then you can save the Access2007 version database into an earlier Version 2002-2003 or 2000, by using the Save As… option from the Office Menu (top left corner menu) and select an appropriate .mdb database format and save the active Database as a new copy.
This will not restore the User-level security settings. You have to restore it manually from scratch by assigning access rights to each and every database object at User-Groups or individual User level.
In either case you must Join the Workgroup Information File (a database with .mdw file extension) from Access2007, to attach your database Users/Groups to Access 2007 to use with your .mdb databases or to implement new security profiles on them.
When you open an .mdb database in Access2007 the Users and Permissions Menu is visible under Database Tools. But, your database Users or User-groups will not be visible under this option unless you Join the Wrokgroup Information File, you were using in earlier version of Access.
In Access2003 there is an option under Tools—>Security to run the Workgroup Administrator program to Create/Join a Workgroup Information File. In Microsoft Access2000 this program is located in the Language Folder (…\1033 for U.S. English) of MS-Office C:\Program Files\Microsoft Office\Office\1033\WrkgAdm.exe to run and Join the Workgroup Information File.
But, in Microsoft Access2007 none of these options are available, except running the following command from VBA:
You can run this command from the Debug Window (Immediate Window, Alt+F11 then Ctrl+G to display) manually or from a Command Button Click Event Procedure and Create a new Workgroup Information File or Join an existing one on Server.
Once you Join the Workgroup Information File you can start using the .mdb files with their User-level security in Microsoft Access2007 or restore the lost security settings manually.