In an expression you need to refer to the column as a Field using Fields! then you give it the field name so Fields!Rank then you get the property, in this case the value so Fields!Rank.value.

Your not writing SQL code in expressions, your writing .Net code.

=IIF(Fields!Rank.Value = 1, 'Gold') Which means that is the field called rank has a value of 1, colour it gold.