That's not clear.  我不清楚。

I can't see your point. 我不明白你在说什么。

I'm not sure what you mean.我不明白你的意思。

I don't have any idea what's going on. 我压根不知道是怎么回事。

It's over my head.  太难懂了。

It's beyond me. 我完全不能理解。