But I have no idea how to calculate the test statistic. I ended up with 4/100 observations which met the criteria but I am confused how to go on frm here.

The simulated data has 100 fields with mean 0 and standard deviation 0.5.

Is it simply 0.45? since that is the 4 observations above 0.45?

You can repeatedly calculate the mean difference with 10 plots (say, 10,000 times). That would give you a distribution, and then it's easy to calculate a p-value: just calculate the proportion of simulations in which the mean =< 0.45%.

However I think there is an error in the text. It says that the difference between the two measurements is normally distributed with mean 0.5. In fact that should probably be a mean of 0 and a standard deviation of 0.5.