You don’t log in there, you visit <your instance’s url>/c/<the community you want>@<url of that community’s instance>

Like this: https://lemmy.fmhy.ml/c/[email protected]

You could also search for !<the community you want>@<url of that community’s instance> in your instance’s search field. In this example, you would put “[email protected]” in your instance’s search field.