1.8 and 1.9 have a completely different structure when it comes to models, which is why your models will work in 1.9 and 1.10 as well as any version above (unless they choose to change the model structuring AGAIN), but won't work in 1.8.
^ That basically means: Yeah, Cubik Studio doesn't work for 1.8.
Finally, when people say "Use Cubik PRO 1.8" and "There are 2 .exe's", they're talking about the old version of Cubik; Cubik PRO. There were 2 .exe's, one for 1.8 models and one for 1.9 and above models. No, you didn't buy the program, no, there's nothing you should or need to change, 1.8 just isn't supported anymore.
Hope that clears things up!