thank you, it worked!!

I will upload the new version

There's 2 ways if your own CSS gets overwritten by a different one:
1. use display: none !important;
but the use of !important should be avoided whenever possible ...
2. or in this case use a more specific rule like

.dropdown-menu .menu-topbar-dropdown-additionalmenu {
    display: none;
}

now it is up to date

I only updated it on my computer. I will upload the newest file in a few minutes.I’m

Just downloaded your code and in css it shows following maybe wrong file you uploaded?

.additionalmenu {
    display: none;
}
/* CSS for mobile devices (480px) */
@media only screen and (max-width: 480px) {
    .additionalmenu {
        display: block;
    }
}

I changed my css to

.menu-topbar-dropdown-additionalmenu {
    display: none;
}

it

shows this

How do I get the component to take my CSS and not the ossn default?

When I delete

css/ossn.default

from

ossn_extend_view('css/ossn.default', 'css/additionalmenu');

in my code, It shows an error "too few arguments"

Your css doesn't applies as Michael said css of original menu entry is different as you can see in image attached by Michael.

Do as advised in https://www.opensource-socialnetwork.org/wiki/view/1137/how-to-find-something-a-word-pattern-in-the-source-code

In this case you'll notice that your CSS is simply addressing a not existing class :)

Actually,

menu-topbar-dropdown-

has been added silently by the Ossn framework to your entry, so you have to adapt your CSS accordingly to make it work.