Thanks for your answers! Finally I managed it to send IC-Prog some data to the programmer. I had to install a special driver for IC-Prog, which allows the software to access the LPT port. Now I see the voltage level rising and falling quickly during the programming. But the CLK line still does nothing. It's clear that the AT89C2051 won't get any data from the buffer-ICs as long as these remain without a clock signal.

The programming voltage at the reset pin seems to be correct: 11,9V during programming, 4,8V during reading and 0V idling.

Now I have two questions:
Do I have to unchek the LockBit boxes in IC-Prog to verify the transmitted data?
What's with the clock signal?

ps: My name is Maxim Walther. I also know that there are better and newer uCs. But I hava to use the AT89C2051, it wasn't my decission. It's one of the criteria which has to be fulfilled in the project, likely due to the low prices.