It's not just Cubase. I just bought Modular V to use with Digital Performer 9 (and other software). AFAICS, there is no "effects" version of Modular V, so you can't use it to process "external" signals. This is extremely disappointing. If anyone has found out otherwise, please let us know. There are numerous complaints about this, but I can find no response from Arturia.
I also have the "Arp2600" plugin, which does have an "Effects" version. I haven't been able to get that to work as an effect either.