You will need something called a level convertor for RS232, which is +-3 to +-12 V rather than unipolar (and also inevrted). The most common is anything similar to MAX232 (for convenience, choose a derivative of it which needs only 100nF capacitors - e.g. MAX202, but other manufacturer may have different markings - e.g. xxx232B or similar - check out the datasheets).

But look at your PG302-it contains one of such. So you can simply use that one if you want, removing the programmer's AT89C2051 out of its socket (at the same time you can use also the power source from that kit).

Or, you might want to build your own "development kit", maybe something similar to http://www.hw.cz/constrc/rd2kit/index_en.html ...

Jan Waclawek