The OpenCL API snippets are avaiable within C/C++ source files.
The OpenCL C Language Syntax and snippets apply only to files with the .opencl extension, this is due to the builtin support for Lisp which has already specified the .cl extension. You can override this by selecting the menu View->Syntax->Open all with extension as...->OpenCL.
This package uses a modified version of the OpenCL.tmLanguage definition available here https://github.com/thebaldwin/OpenCL-Syntax. Thanks for the good work.