I couldn't find any mention of the Mac App Store version in the CS2015 FAQ, which only states this about playing on Macs:
If you do not yet have a Steam account, you can set one up for free.
You will need to download a Steam client to set up an account.
The Steam client is available free of charge at the following website:
Download Steam Client
Please follow the Steam setup prompts and be sure to set up a Steam
In order to activate your game in Steam, please do the following:
Open Steam and log into your account.
Click on the heading “Games” and then the subheading “Activate a Product in Steam…”
Follow the steps to activate the product and enter your product key at the end.
Afterwards, Construction Simulator 2015 will automatically be
downloaded and installed
I was able to confirm the game exists on the Mac App Store, so this seems to be an oversight in their FAQ. Realising this, I contacted the developer via the official Facebook page and relayed your question. This was their response:
Thanks a lot for getting touch. It is unfortunately not possible to transfer the accounts between platforms.
In other words, unless you have a product key you can activate through Steam, you will have to repurchase the game on Steam if you wish to play on Windows.