You're instantiating a new HighScores instance (hScore). Unless the constructor for HighScores populates itself with data (?) there will be nothing to dereference at the following line:

if (GameManager.score >= hScore.highScores[score])

because hScore.highScores (an array or list I'm assuming) hasn't been populated with any content. For all we know, hScore.highScores itself is still null.

how could you manage this for multiple users?

Off-the-cuff here, but a high level version would be if you want the scoreboard to reflect local players only, the game can just load up the external .dat file when you start the game (so the current high scores are in memory), and then save it back out on exit with any changes.

If you want it integrated to facebook and reflecting multiple remote users, that .dat file will need to live somewhere all the FB instances can access it, or be converted to a database table (with interface changes in the code as well as you'd no longer be loading in from a file stream).

If you want it integrated to facebook but still only want to track the local players' high scores, you can save some kind of file off into browser cache/cookies.

There are other solutions as well, my web-stack experience is much shorter than my desktop/enterprise knowledge.

ArgumentException: Object type System.Int32 cannot be converted to target type: System.Int32[]

"An integer can't be converted to an array of integers"
Which line is throwing that error?

There should be more information in the Console tab/window, to include the full stack-trace of the error/output (when you select any entry in the Console list, i.e. one left click). Each entry in the trace should tell you which file it was in for that step, and which line the function call for that step happens at. Resize the console tab to see more (as the stacktrace content appears at the bottom of the window), or just highlight the whole mess and paste it into a notepad or something to read.