The software on your 7025 seems to be a very old one - try to update it.
I suppose you use the actual Version of DreamUp from http://www.dream-multimedia-tv.de/download/DreamUP.zip
Are you sure that your nullmodem cable is OK and you selected the correct COM port in DreamUp?
To make a basic test, connect PC and Dreambox via the nullmodem cable, open Hyperterminal, create a new connection by selecting the correct COM port (115200 baud, 8 data bits, 1 stop bit, no parity, no handshake/flowcontrol) and turn on the Dreambox.
You should see some output from the bootloader, otherwise your cable is not OK or you selected the wrong COM port.
If this test was sucessfull you can start using DreamUp - either with or without network (though i recommend using it - otherwise flashing takes about 45 mins).
When you use DreamUp without network, it's normal that you can't ping the box as there is no network active to handle it