How do I set the visibility of a text box in SSRS using an expression?

=IIf((CountRows("ScannerStatisticsData")=0),False,True)

Should be replaced with

=IIf((CountRows("ScannerStatisticsData")=0),True,False)

because the Visibility expression set up the Hidden value.

This didn't work

=IIf((CountRows("ScannerStatisticsData") = 0),False,True)

but this did and I can't really explain why

=IIf((CountRows("ScannerStatisticsData") < 1),False,True)

guess SSRS doesn't like equal comparisons as much as less than.

I tried the example that you have provided and the only difference is that you have True and False values switched as @bdparrish had pointed out. Here is a working example of making an SSRS Texbox visible or hidden based on the number of rows present in a dataset. This example uses SSRS 2008 R2.

Step-by-step process: SSRS 2008 R2

In this example, the report has a dataset named Items and has textbox to show row counts. It also has another textbox which will be visible only if the dataset Items has rows.
Right-click on the textbox that should be visible/hidden based on an expression and select Text Box Properties.... Refer screenshot #1.
On the Text Box Properties dialog, click on Visibility from the left section. Refer screenshot #2.
Select Show or hide based on an epxression.
Click on the expression button fx.
Enter the expression =IIf(CountRows("Items") = 0 , True, False). Note that this expression is to hide the Textbox (Hidden).
Click OK twice to close the dialogs.
Screenshot #3 shows data in the SQL Server table dbo.Items, which is the source for the report data set Items. The table contains 3 rows. Screenshot #4 shows the sample report execution against the data.
Screenshot #5 shows data in the SQL Server table dbo.Items, which is the source for the report data set Items. The table contains no data. Screenshot #6 shows the sample report execution against the data.