Edit: Sorry, I completely spaced and didn't realize that you had already supplied that information (Debian 4.02) X-D.
There are no specific instructions for Debian, and it turns out that there isn't a Debian specific package for NDISwrapper, which means that we'll have to do this the (not too) hard way. Download the source tarball (the nickname given to compressed files of the *.tar.gz or *.tar.bz2 extension) from here
Now, go to a command prompt (either by pressing Ctrl+Alt+F2 in KDE or selecting the GNOME terminal emulator from the application menu in the bottom left corner of your desktop) and change directories to wherever the source tarball is saved, using the cd command like so:
cd (directory where the tarball is, probably /home/username/Desktop).
Then, you do what is called untarring, or extracting, the file like so:
tar -xzf ndiswrapper1.51.tar.gz (or whatever version you get, use the name of the file you downloaded).
Now, this should have created a new directory called just ndiswrapper1.51, the name of the file you downloaded without the .tar.gz extension. change into said directory using cd again.
Once there, you will first want to READ the README and INSTALL files. Always always ALWAYS read these files before you compile/install anything, as they often contain great info about the program you are about to use and will usually answer questions you might otherwise have to ask here.
It should be fine to then issue the configure command, which will generate things called Makefiles, which tell your computer how to compile the program correctly. It's easy because you only have to tell you comp to do it, not how to :-P. So type
and it should spit out a lot of stuff onto your terminal. If nothing goes wrong (i.e. the output doesn't say something like missing package so and so) then we are go for compilation. Type
to compile your very own version of NDISwrapper. This might take a minute or two, so be patient. Once all the numbers and flashy letters stop scrolling by, and the last few lines of output don't say ****ERROR****, then you got a good binary and its time for the next step. Change to super user (root) by issuing the
command and typing your root password. Note that that is your root password and definitely should be different from your user password.
and NDISwrapper should be put where it wants to go. Once you have gotten this far, congratulate yourself! You've just installed a program from source code, a not so insignificant accomplishment. Please reply when you have completed these steps successfully and we'll continue.
Happy Hunting :-)