I have not used these cells but I have read a lot about them. DT40 cells is a B cell line from avian leukosis virus induced bursal lymphoma in a white leghorn chicken.
From what I have read this is a difficult cell line to transfect. Both Lipofectamine and FuGene don't work very well most people seem to electroporate if they want to transfect. Electroporation seems to me the be the simplest choice and the bottom reference has electroporation conditions and so do other articles I looked at.
Buerstedde J.-M. and Takeda, S. (1991) Increased ratio of targeted to random integration after transfection of chicken B cell lines Cell 67, 179-188.
I found this web site which happens to be a very good reference for growing and manipulating DT40 cells http://cwp.embo.org/pc10-17/docs/Sale.pdf