Please folow these steps (when have national pokedex)
1.fly to the pokemon legue
2.go to victori road
3.if you have an super rod now you have the national pokedex the persen that was bloking the tunnle he is gone now (if dont go to the battle place but dont go in the battle areafind a fisherman talk to him then he will giv you an super rod)go thru the tunnle then there will be a thing go thru g it then you will go with an girl thru it but dont go thru the hole intier cave find a fishing place and there you go you will need20 dusk balls good luke