To create a modal:

1- Navigate to Modal > Add New.
2- Add your title and the content.
3- You can change the Modal size from the Modal options meta box (Small, Medium or Large) Examples.
4- Publish your modal.

You can assign a modal to header menu link or button.

1- Navigate to Modal > All Modals and copy the Button URL.
2- Navigate to Appearance > Menus and select the header menu you want to work with.
3- You have to be sure that Screen Options (CSS Classes) box is checked.
4- Click on Screen Options.
5- Check CSS Classes box.
6- Add the word modalbtn to CSS Classes (optional) box.
7- Add the Button URL you have copied from step 1 to the URL textfield.

If for some reason the modal slug has a conflict with other plugins, you can change the slug from Plume Core options > Modal > Slugs.

Navigate to Plume Core > Modal > Slugs and change the main slug modal-items to anything else that doesn’t has a conflict.

http://plume.graphicfort.com/modal-items/large-modal/

The modal-items is used for the modal slugs and you have activated a plugin that uses modal-items as a slug, so you will need to change modal-items to anything else.

EX:

http://plume.graphicfort.com/modal-posts/large-modal/

after that you will have to navigate to Settings (From the WordPress admin left bar) > Permalinks and click on save button.