It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.entity.AttributeInstance;

ZenGetter	GetterMethod	Return Type
ZenGetter attribute	GetterMethod getAttribute()	Return Type IEntityAttribute
ZenGetter baseValue	GetterMethod getBaseValue()	Return Type double
ZenGetter modifiers	GetterMethod getModifiers()	Return Type List<IEntityAttributeModifier>
ZenGetter attributeValue	GetterMethod getAttributeValue()	Return Type double

ZenSetter	SetterMethod	Parameter Type
ZenSetter baseValue	SetterMethod setBaseValue(value)	Parameter Type double

• List<IEntityAttrubuteModifier> getModifiersByOperation(int operation)
• boolean hasModifier(IEntityAttributeModifier modifier)
• IEntityAttributeModifier getModifier(String uuid);
• void applyModifier(IEntityAttributeModifier modifier);
• void removeModifier(IEntityAttributeModifier modifier);
• void removeModifier(String uuid);
• void removeAllModifiers();