I'm trying to use BC pump for lava, but universal cable won't connect to it.

(Minecraft 1.7.10)
Mekanism-1.7.10-7.0.0.75.jar
buildcraft-6.0.17.jar

Already reported - see #1610

Presumably will be fixed in the next release. Until then for a workaround you can connect a wooden kinesis pipe to an energy cube and then run BC power pipes to the pump.

@urmet is correct, I've already coded, but do need to merge in the update to BC 6's API. This should come some time soon.