Well, you have to compile the code.
You can try use the Keil C-compiler, and use the C51 compiler option. http://www.keil.com/c51
C-code is human readable, and what a compiler does is to convert this code and make it machine readable, like ones and zeroes 
I don´t have to much experience with 8051 controllers, so give it a try, read some guides about it 