the Powertech Encryption field procs are called automatically by DB2 when a job accesses a field (column) that has the field proc associated with it, i.e. a column that is activated in Powertech Encryption. In that sense, there is no need to call it explicitly --- calls happen automagically.
The product does ship with APIs that you can call, e.g., from Stored Procedures. You can use this, e.g., to use criteria other than the current user to control access to encrypted data. These APIs are not field procs, but other programming interfaces to the product's functionality.
It's probably best if you let us know if you are looking for a specific functionality, or if you want to explore what is available in terms of APIs.