For Buildcraft waterproof pipes: Place a wooden waterproof pipe onto the lava drum - this should extract the lava from it. Use other waterproof pipes of your choosing to carry it from there to the generators.
For Thermal expansion fluiducts: connect the drum to the generators with fluiducts. Then, use a wrench to right-click on the connection onto the drum until it displays a dull red arrow pointing away from the drum. Apply a redstone signal and the red arrow should light up and lava will begin to flow from the drum.
For Ender IO fluid conduits: connect the drum to the generators using conduits. If you're not using the pressurised conduits, make sure that the flow is never travelling upwards. Then, right click on the connection onto the lava drum (the wide square bit). This should bring up a GUI. Select extraction mode, and then either set it to not require a redstone signal or apply a redstone signal. Lava should flow from the drum to the generators.
For Extra Utilities transfer pipes: Place a fluid transfer node onto the drum, and run transfer pipe from there to the generators.
Don't know which method you're using, so I've covered them all
If someone knows more than I do (especially with regards to Buildcraft and Extra utilities) then feel free to correct me
