Brand name OEM motherboards do not have options to do what you want to do. You need to replace it with a motherboard that will give you the option to do so.
Also not all Sempron 140 CPUs will allow to to successfully unlock the 2nd core. The 2nd core was locked by AMD because it did not pass certain tests after the manufacturing process. Generally speaking one of three things can happen when you attempt to unlock the core:
#1 - Successful unlock. In general it works fine , but it may not perform as well as a dual core Sempron / Athlon of the same speed.
#2 - While unlocked, the 2nd core may cause your PC to become unstable because of many small defects.
#3 - There are so many defects with the 2nd core, it may not be possible to unlock and use the 2nd core.