I have just programmed a Cisco PIX501 and want to use PDM?

I have just programmed a Cisco Pix 501 with basic settings but no pwords as yet. I want to use the PDM but it asks me for a username and pword twice. The first time I just hit enter and it takes me to the next stage, it asks me again for username and pword but when i press enter it say authentication failed. Can anyone tell me what the default pword and usename is or do I have to enter such things first to be able to use the PDM. I get the PDM splash screen etc.