This has been asked hundreds of times before at CSC, please search for answers before posting.
The possible solutions and workarounds are also contained in all the previous threads if you're interested in them, just do a quick search to find all that info.
CUCM does not have a way to password protect a meetme conference. However, there are a few work arounds that you can do to accomplish this. You can use Unity Connection or UCCX. Unity connection will not prompt for a pin, but it will allow you to accept or deny entry into the conference. UCCX you can script it to allow for a pin. Here is the Unity Connection way.
MeetMe number is 1111 and CTI RP is 2222
Create your meetme conf 1111
Creat a CTI RP with extension 2222 and CfwLL to voicemail
Create the First Callhandler MeetMe-Transfer and under transfer rules:
Create a Second CallHandler MeetMe-Welcome with 2222 extension and customize the greeting to say : Please press 1 to join the conference and under CallerInput make sure to transfer to the MeetMe-Tranfer.
Look at this link for the UCCX Way