Okay, let's see, maybe written instructions help better:
First, you click on the icon on the right, which says ツーしん on your pocket designer (with the heart).
Then you choose たまごっちP's, then あげる (which means give). Then you select the category. You want to give the dress, so search for ようふく (clothes). Search for the dress and press okay, which is けってい.
Then pick up your P's, go to the connection menu (the heart), go to たまごっちP's. Choose the option アイテムこうかん (item transfer), then choose もらう (receive - on your designer you give, on your P's you receive).
Hope that helps.
Ask if you have questions!