There are two cases I can think of why the Format function is not defined.
First one is if you are using a version of Telerik Reporting prior to Q1 2008 - this was the version in which the function was implemented. So make sure you are using the latest version of the product.
Second one (more likely), if you are trying to use the function to format more than one value. At this time the Format function supports only the following signature:
System.String Format(System.String format, System.Object value)
If the above is not relevant to your case please open a support ticket and send us a sample report that demonstrates the described behavior.
the Telerik team