Hi, hdl actually is not supported in Arduino ide. For that you have to work directly with quartus and you can simulate it using modelsim, shipped with quartus installation. This is not something for beginners and I suggest you go down this path only if you are really determined. On Intel web site there are lots of tutorials and free online training courses about almost any aspect of quartus and modelsim however the learning curve is a bit steep...
As you probably read we are actually trying to make this easier by providing ready made IP blocks and project templates.
At the moment we're not providing an easy way to simulate a custom block but this for sure is something we may work on in the near future.
Thanks,
Dario