In order to add more functions to the "functions" drop-down in the Fields and other tabs, you use the extended functions setting. In your global.asax file, inside of ConfigureSettings(), place this code:

AdHocSettings.ExtendedFunctions = new string[] { "STDEV","LOG"};

The setting uses an array of strings, each of which correspond to built-in SQL functions. In this example, we have added the log function and standard deviation function to the drop-down. The will now be available for selection in the report designer.