Well it is having trouble because it can not create or over write this file...
c:\Program Files\Microsoft SQL Server\MSSQL10_50.SQLEXPRESS\MSSQL\DATA\arcadia8.1 .mdf
Although it should be arcadia8_1.mdf
Make sure in the restore options you are checking the box for "Overwrite with replace"
If that still fails delete the file and let it create a new one.
c:\Program Files\Microsoft SQL Server\MSSQL10_50.SQLEXPRESS\MSSQL\DATA\arcadia8.1 .mdf
Although it should be arcadia8_1.mdf
Make sure in the restore options you are checking the box for "Overwrite with replace"
If that still fails delete the file and let it create a new one.