The Inline function, IF is not available in the K2 Five Designer under Context Browser > Functions > Logical.
This is by design, HTML Designer is a new product and no feature parity was done between Silverlight and HTML.
If you upgraded from K2 4.7 to K2 Five, legacy tools will be available for you and the IF function will be available in K2 Studio and in Silverlight designer.
We have an existing TFS created for this issue which is currently under investigation to include this functionality in a future release of K2 i.e. K2 Five 5.2.