Make sure that the account under which SQL Server service runs has access to the path under which MDF and LDF files are present.
Also try the steps below from SSMSE:
1. Right Click Databases in SSMS
2. Choose Restore Database
3. Provide Database name in which it needs to be restored
4. Choose From Device and then provide the path of your bak file
5. Go to options and then check whether the MDF and LDF file names and path are ok and not conflicting with any existing files (If conflicting provide some other path or filename) and click OK
