[SalesForce] open account new detail page on click of a button

In managed packages, you can add new page layouts, but you cannot upgrade existing page layouts, including layouts that are part of the package..

More info on what you can create and upgrade here: https://org62.my.salesforce.com/help/pdfs/en/salesforce_packaging_guide.pdf

From the docs:

So that "New" button is a feature called "Quick Create" It's an old old old feature.

Right now, it does not support custom objects. You can vote on the idea here: https://success.salesforce.com/ideaView?id=08730000000BqPaAAK

something to consider is that Quick Create will ignore any validation rules you have on those objects. Many admins frown upon it's use because it can cause a lot of bad or incomplete data to be created in the system.

Some options you can consider instead of having them go to a different tab to create:

1. Look into creating Quick Actions (Chatter Publisher Actions) to create the records
2. Replace all your pages with visualforce pages so you can create your own version of quick create (Don't do this! Terrible idea but I wanted at least 2 items)

Quick actions are really your best bet but you can't do them from the lookup dialog window. But it would save them from having to leave the record they're viewing.