I have dlink dir600 on my computer and forgot the password and can not log on with my iPhone

The default password is left blank on that modem model. If you've set a password and can't remember it, you could restore the modem to factory default, or just change the password. To do either of those, navigate to the following address:

http://192.168.1.1 if that doesn't work, then try http://192.168.0.1

It will most likely ask for your user name and password. The default user name is: admin
And like I said, the default password is nothing, it's left blank. If you set a password, and have now forgotten it, then you'd need to do a hard reset. There should be a place in the back of the modem in which you can stick a pen or something similar in, and press and hold for a few seconds and it should force your modem to reboot with default settings.