loginitem

+ (BOOL) willStartAtLogin:(NSURL *)itemURL
{
    Boolean foundIt=false;
    LSSharedFileListRef loginItems = LSSharedFileListCreate(NULL, kLSSharedFileListSessionLoginItems, NULL);
    if (loginItems) {
        UInt32 seed = 0U;
        NSArray *currentLoginItems = [NSMakeCollectable(LSSharedFileListCopySnapshot(loginItems, &seed)) autorelease];
        for (id itemObject in currentLoginItems) {
            LSSharedFileListItemRef item = (LSSharedFileListItemRef)itemObject;

            UInt32 resolutionFlags = kLSSharedFileListNoUserInteraction | kLSSharedFileListDoNotMountVolumes;
            CFURLRef URL = NULL;
            OSStatus err = LSSharedFileListItemResolve(item, resolutionFlags, &URL, /*outRef*/ NULL);
            if (err == noErr) {
                foundIt = CFEqual(URL, itemURL);
                CFRelease(URL);

                if (foundIt)
                    break;
            }
        }
        CFRelease(loginItems);
    }
    return (BOOL)foundIt;
}

+ (void) setStartAtLogin:(NSURL *)itemURL enabled:(BOOL)enabled
{
    OSStatus status;
    LSSharedFileListItemRef existingItem = NULL;

    LSSharedFileListRef loginItems = LSSharedFileListCreate(NULL, kLSSharedFileListSessionLoginItems, NULL);
    if (loginItems) {
        UInt32 seed = 0U;
        NSArray *currentLoginItems = [NSMakeCollectable(LSSharedFileListCopySnapshot(loginItems, &seed)) autorelease];
        for (id itemObject in currentLoginItems) {
            LSSharedFileListItemRef item = (LSSharedFileListItemRef)itemObject;

            UInt32 resolutionFlags = kLSSharedFileListNoUserInteraction | kLSSharedFileListDoNotMountVolumes;
            CFURLRef URL = NULL;
            OSStatus err = LSSharedFileListItemResolve(item, resolutionFlags, &URL, /*outRef*/ NULL);
            if (err == noErr) {
                Boolean foundIt = CFEqual(URL, itemURL);
                CFRelease(URL);

                if (foundIt) {
                    existingItem = item;
                    break;
                }
            }
        }

        if (enabled && (existingItem == NULL)) {
            LSSharedFileListInsertItemURL(loginItems, kLSSharedFileListItemBeforeFirst,
                                          NULL, NULL, (CFURLRef)itemURL, NULL, NULL);

        } else if (!enabled && (existingItem != NULL))
            LSSharedFileListItemRemove(loginItems, existingItem);

        CFRelease(loginItems);
    }       
}

If you want an easy to implement checkbox, make a @property BOOL startAtLogin; in one of your classes and implement it as follows. Just bind the checkbox value to the property and it should all work seamlessly.

- (NSURL *)appURL
{
    return [NSURL fileURLWithPath:[[NSBundle mainBundle] bundlePath]];
}

- (BOOL)startAtLogin
{
    return [LoginItem willStartAtLogin:[self appURL]];
}

- (void)setStartAtLogin:(BOOL)enabled
{
    [self willChangeValueForKey:@"startAtLogin"];
    [LoginItem setStartAtLogin:[self appURL] enabled:enabled];
    [self didChangeValueForKey:@"startAtLogin"];
}