I don't remember my bios password. so what can i do to reset this password ?

When you say bios password, do you mean firmware password? If so, the only way to reset the firmware password is to take the MacBook to an Apple genius bar and have them reset it.

What bootable CD are you refering to? The original 10.6.x install DVD or something else.

You should be able to boot from the install DVD by holding down the c key at start up or holding the option key and then selecting the disk.

If you boot from the install DVD you can reset the administrator password by going to the utilities menu.

If the OP means firmware password instead of bios password, then it is not possible to boot from the DVD without the password...that is the purpose of firmware password protection.

If you have a mac with removible ram, you can try doing a hardware hack. If your computer's ram is soddered to the motherboard, you will need to take it someware like an apple store or Mac Life. Hear are the steps you can try performing if your computer has removible ram. Turn off your macbook, and unplug everything from it, including the power cord. Note, if you have been using your computer, after you turn it off, wait 5 to 10 minutes to let the components cool off, but you don't have to do this, Apple just suggests you do it, sence the components can get pretty warm the longer you use it. Remove all of the screws holding on the bottom case, then carefully lift it and set it aside. Touch a mettle object to discharge any electricity from your body. Now, remove all of the screws holding on the l bracket, and then remove it. Now, carefully push the levvers away from the ram sticks until 1 pops out. Carefully pull the ram card out and set it aside, you will put it back when you finish working on your computer. Leave the bottom ram stick in the ram slot, because your computer can't function without ram. now, press and hold the option, command, p, and r keys, then turn on your computer, but continue to hold those 4 keys down. Wait until the mac's startup chime sounds 4 times, then turn your computer off, you have successfully reset your forgotten firmware or bios password. Now, reinstall the ram stick you removed back into the slot, making sure it clicks and snapps into place. Now, reinstall the l bracket, reinstalling all the screws that holding it on. Now, replace the bottom case, and reinstall all of the screws you removed when you opened your mac's bottom case. If this tip didn't work for you, such as if your mac's ram is soddered to the motherboard, or if you completed all these steps and you weren't able to reset your forgotten firmware or bios password, The only other option you have is to take your mac into an apple store, mac life, or another mac shop, and have them reset the password for you. Hoped this helped, smiley.