It appears that the load test repository was not created for some reason, so you will need to run the script to create it. By default, the load test attempts to run against the SQL Server Express database that is installed with VS. If this database is installed then do the following:
1) Open a cmd prompt
2) Change into the following directory: <VS Install Dir>\Common7\IDE
this will be something like: D:\Program Files\Microsoft Visual Studio 8\Common7\IDE
3) Run the following command: sqlcmd -S .\SQLExpress -i loadtestresultsrepository.sql
Then test the connection the way you originally did with the Administer Test Controller dialog.
If you do not have SQL Express installed on your local machine, you will need to run the installation script on a sql server instance and then configure the run to use the sql server you setup. The installation script is called loadtestresultsrepository.sql and is located in the <VS Install Dir>\Common7\IDE directory. After running this script, do the following:
1) Open the Administer Test Controller dialog.
2) Click the ... next to the connection string
3) Point the connection to the database you just installed
4) Test connection and hit OK