To Import MSSQL Databases in Plesk  please follow the following steps :-

1) Login to Plesk control panel.

2) Click on "Websites & Domains".

3) Click on "BackupManager".

4) Click on "Database Backup Repository".

5) Click on "Upload Backup File."

Note : You can upload only .bak file only.

6) Click on Browse to select the .bak file from your Local computer.

7) Click Ok.