Hello, I am running MineTweaker 1.7.10-3.0.1.0, ModTweaker 0.9.5 and Mekanism 8.1.8. I am currently trying to use arrays of Mekanism gases. To do this, I need to import the modtweaker.gas.IGasStack Zen class (aka the modtweaker2.mods.mekanism.gas.IGasStack interface).

Unfortunately, attempting to do that in ZenScript (with a line like import modtweaker.gas.IGasStack;) fails with just the following line being printed to chat and the Minetweaker log:

ERROR: ?:1 > could not find package null

The same thing happens with modtweaker.gas.IGasDefinition too, I haven't tested much else.

Importing other classes (such as mods.mekanism.chemical.Dissolution) works well.

I don't know much about the Minetweaker API, but I think this happens because these interfaces are not registered with MineTweaker. Registering them together with the other Mekanism classes in modtweaker2.mods.mekanism.Mekanism could help.

Could you link your used ZenScript?

Just the following line is enough:

import modtweaker.gas.IGasStack;

Yes i think registering should solve this error.
Actually i was interested why you use this import statement, because for normal recipe modify this is not necessary.

As I have stated in the OP, I am using this so that I can have arrays of gases. I use them to simplify creation of a large number of recipes to better integrate Mekanism with GregTech. I may also need to pass gases as function parameters.

Edit: I've just disassembled the Mekanism.class file, added the appropriate registerClass call, assembled it, replaced the original one in the ModTweaker jar and it seems to have done the trick.