Changing the package to main.java did the trick.

In the java class that is:

package main.java;

Whether the import statement is updated or not doesn't matter, both work:

(:import [main ModelLoader])

and

(:import [main.java ModelLoader])

Why? I wouldn't mind a more in in-depth explanation myself.

Now with a working package the class visibility pointed to by @Eugene actually makes a difference: the class needs to be public. Otherwise the same error persists.