Dear yusufkocak,
You can use the Epanet 2 toolkit, including the two files: Epanet2.dll and Epanet2.bas. Copy the two files to the directory of your vb6 application's directory.
You can find the toolkit here:
http://www.epa.gov/nrmrl/wswrd/epanet.html#Downloads
For a simple calculation (i.e. no controlling of the calculation process) you can use the ENepanet function.
I have an an vb6 example for you, if you are interested.
Hope this helps!
regards,
Eelco Trietsch
Kiwa Water Research