Hello! You can access the nightly builds from within Steam! To learn how to, you can check our documentation here: https://ar14.works/mannequin-docs-gettingstarted
Here's the part from the documentation which explains the process:
You can also switch to the Nightly builds, which are more frequently updated with in-progress features but are potentially less stable, using the Properties window. To access this, click the gear icon on the left side of the library view. In the Properties window that shows up, switch to 'Betas' tab and choose 'nightly' in the drop-down menu.
Hope this helps. Let us know if there's still problems!