Install "node-gyp", compile with "node-gyp configure build", voila.

binding.gyp

{
  "targets": [
    {
      "target_name": "keypress",
      "conditions": [
	      ['OS == "mac"', {
		      'include_dirs': [
		          'System/Library/Frameworks/CoreFoundation.Framework/Headers',
		          'System/Library/Frameworks/Carbon.Framework/Headers',
		        ],
		      "link_settings": {
	                            "libraries": [
	                                "-framework Carbon",
	                                "-framework CoreFoundation"
	                            ]
	                        }
	            }
	          ]
	        ],
	      "sources": [ "keypress.cc" ]
    }
  ]
}

keypress.cc

#include <node.h>
#include <v8.h>
#include <stdio.h>
#include <stdlib.h>

#ifdef OSX

#include <CoreFoundation/CoreFoundation.h>
#include <Carbon/Carbon.h> /* For kVK_ constants, and TIS functions. */

#endif

#ifdef GTK
// hmm. soon.
#endif 

 
using namespace v8;

#ifdef OSX

CFStringRef createStringForKey(CGKeyCode keyCode)
{
	TISInputSourceRef currentKeyboard = TISCopyCurrentKeyboardInputSource();
	CFDataRef layoutData = (CFDataRef) TISGetInputSourceProperty(currentKeyboard, kTISPropertyUnicodeKeyLayoutData);
	const UCKeyboardLayout *keyboardLayout = (const UCKeyboardLayout *)CFDataGetBytePtr(layoutData);

	UInt32 keysDown = 0;
	UniChar chars[4];
	UniCharCount realLength;

	UCKeyTranslate(keyboardLayout,
	               keyCode,
	               kUCKeyActionDisplay,
	               0,
	               LMGetKbdType(),
	               kUCKeyTranslateNoDeadKeysBit,
	               &keysDown,
	               sizeof(chars) / sizeof(chars[0]),
	               &realLength,
	               chars);
	CFRelease(currentKeyboard);    
	return CFStringCreateWithCharacters(kCFAllocatorDefault, chars, 1);
}

CGKeyCode keyCodeForChar(const char c)
{
	static CFMutableDictionaryRef charToCodeDict = NULL;
	CGKeyCode code;
	UniChar character = c;
	CFStringRef charStr = NULL;

	if (charToCodeDict == NULL) {
	    size_t i;
	    charToCodeDict = CFDictionaryCreateMutable(kCFAllocatorDefault,
	                                               128,
	                                               &kCFCopyStringDictionaryKeyCallBacks,
	                                               NULL);
	    if (charToCodeDict == NULL) return UINT16_MAX;

	    /* Loop through every keycode (0 - 127) to find its current mapping. */
	    for (i = 0; i < 128; ++i) {
	        CFStringRef string = createStringForKey((CGKeyCode)i);
	        if (string != NULL) {
	            CFDictionaryAddValue(charToCodeDict, string, (const void *)i);
	            CFRelease(string);
	        }
	    }
	}

	charStr = CFStringCreateWithCharacters(kCFAllocatorDefault, &character, 1);
	if (!CFDictionaryGetValueIfPresent(charToCodeDict, charStr,
	                                   (const void **)&code)) {
	    code = UINT16_MAX;
	}

	CFRelease(charStr);
	return code;
}

#endif
 
Handle<Value> callKeyEvent(const Arguments& args) {
    HandleScope scope;
    
   if (args.Length() < 1) {
        ThrowException(Exception::TypeError(String::New("Wrong number of arguments")));
        return scope.Close(Undefined());
    }
 
    if (!args[0]->IsString()) {
        ThrowException(Exception::TypeError(String::New("Wrong arguments")));
        return scope.Close(Undefined());
    }
 
    Local<Object> obj = Object::New();
    // obj->Set(String::NewSymbol("x"), Number::New( 1 + (rand() % xBound->IntegerValue() )));
    // obj->Set(String::NewSymbol("y"), Number::New( 1 + (rand() % yBound->IntegerValue() )));

    Handle<Value> arg = args[0];
  	String::AsciiValue value(arg);
#ifdef OSX
    CGKeyCode k = keyCodeForChar( (char) (*value)[0] );

    CGEventRef e = CGEventCreateKeyboardEvent (NULL, k, true);
    CGEventPost(kCGSessionEventTap, e);
    CFRelease(e);
#endif
    return scope.Close(obj);
}

void init(Handle<Object> target) {
    target->Set(String::NewSymbol("callKeyEvent"),
        FunctionTemplate::New(callKeyEvent)->GetFunction());
}
NODE_MODULE(keypress, init)

server.js

var myaddon = require("./build/Release/keypress.node");
console.log( myaddon.callKeyEvent('a') );