The AST Transformations need to be compiled before your project code. The simplest way to do this is to hook into the grails compile event with a script. Check out this blog post for how to create a script with new ant task to precompile source in src/ast folder. http://reinhard-seiler.blogspot.com.au/2011/09/grails-with-ats-transformation-tutorial.html
If you only have a few AST Transformations then this is by far the best approach. Creating a plugin or separate project with compiled jar is too much work for my needs.