How can I convert a 2 prong connector to USB?
This calls for an FTDI FT245R. FTDI has become the standard for interfacing USB in a simple way to microcontrollers and other digital I/O.



The FT245R has 8 parallel I/O, so you can connect the switch with a pull-up resistor to one of those. 

edit
You asked about this module:



Looks good. You connect the switch as follows:  

  

VCC goes to VCCIO, Vout goes to one of the inputs D0..D7. I wouldn't solder directly to the module, but use a socket like this instead:  

  

That's it. The advantage over the RS232 interface is that you have 8 I/O at your disposition.  

The yellow jumper on the board selects between 3.3 V and 5 V for the I/O. This is important if you want to connect to a microcontroller or other external logic, but for the switch it doesn't matter.