How can I include groovy script from an external file?

I was tried to use:
def script = new GroovyScriptEngine('d:/soapui/payment.v2').with {
loadScriptByName( 'proxy.groovy' )
}
this.metaClass.mixin script
But I get:

Update
Is there exists some possibility to pack my methods into jar or something like this, and use them from Script TextArea?
The simplest way is to use Groovy Test Step within SOAPUI to run the external script using the GroovyScriptEngine. I use GroovyUtils to find the Project’s path so that the entire project can be kept in one place to aid source control, editing etc.