There are two senearios where it could have an problem updating ,
1. There should be any incompatible software in the system which blocks the DAT downloads to the software, 2.Might be due to malware infections.
Please follow the below prompts:
Check and delete if you have any other security software in C:/program files and common files.
Click on "Start" and then click on "Run". Then Type in "Drivers".
On the window that opens up try to open up the folder named "ETC". then look for the file "Hosts" and then right click on the file and open it with "NotePad".Check the bottom of the page just below from where it says 127.0.0.1 local host.If you have list of websites remove them all and ensure you have the details only up to 127.0.0.1 information about websites below this can be removed.
Once done save the files and try to update McAfee.