This problem arises when you're trying to restore your database to the folder that already contains your MainBoss database. Since you're doing the restore operation with the MainBoss software running, your MainBoss database is busy and none of the files can be overwritten.
There are two ways to solve the problem:
To do a command line restore operation, open a Windows "command prompt" window and use the CD command to go to the folder that contains your MainBoss software. The command to do this will likely be one of the following:
cd "\Program Files\MainBoss\2.9" or cd "\Program Files (x86)\MainBoss\2.9"
Now type in and execute a command with the following form:
mboss restore /MZH:"Backup File Name" /mbdb:"database folder"
For example, if the backup file is C:\MyBackups\2013Jan01.mzh and your database folder is C:\MainBoss, the command would be:
mboss restore /MZH:"C:\MyBackups\2013Jan01.mzh" /mbdb:"C:\MainBoss"
This will restore the database from the given backup file to the given folder. If the folder already contains database files, you'll be asked to confirm the operation before MainBoss actually does it.