We are only going to build ONE Pull-down menu item in this exercise.
Decide the Name of the section, and the 3 names of the subsections that correspond to that section-menu.
Convert each one of them to a functioning BUTTON.

Once you are INSIDE the Movie clip divide the buttons, each one in their own layer:
The section name in the lower layer,
Each one of the subsection buttons, with the layer named as themselves.
Make a layer for Actions.

Move the 3 subsection keyframes to frame 5 (we don't want to see them at the very beginning)

Add frames, at least to frame 15.
Actions layer: Make a Stop() action in frame 1.
Make a frame label called "Roll" on the frame where all the subsections are set to begin.

As you can imagine, on rollOver of the section button, the play-head is going to come to the Frame-label Roll,

and we are going to see the 3 items appearing on screen. Later on we will make a mask for this, but right now you need to set up the animation of those submenu items, tweening from a place ON TOP of the section name, to their final placement under the section name. See the animated gif for this step:

You need to insert a Stop() action at the end of your rollOver animation (Actions layer)
You need to attach the action to the section button, so when the user rolls over the section name the animation bringing the subsection menus occurs. See the action syntax below.

Each one of the elements is a button with rollover color, when you rollover "portfolio" the animation triggers.
The action still doesn't work really smoothly, but the action is in place.

There is still a very important part of the functionality of the pull down menu that is still not in place: As you saw in the test, every time that I rollOver on the menu the animation begins again, but when I get away of the movie I am not coming back to the first position (frame 1: only section name). When I have several pull down item in place, I NEED my menu to "unroll" (to disappear) when I roll on another item.

I am going to create a button, that takes us OUT of the submenu content if we try to move the cursor towards another menu item. For that I need to know WHERE the other submenus are going to be placed, and in this case the other menu-items will be placed inline horizontally with portfolio.

Describing the area of this button is really a test&fix process, I am drawing it this way:
Area on both sides of the button, descending area towards the content with a bell-shape (you want to be able to move your cursor towards the content without getting out of the section, but count with undecisive users, shifting to another section halfway down the Stage)

Drawing
the
area

Delete the shape from the Up State and go directly to make a keyframe in the Hit-area.
(this is how you make a transparent button)

REMEMBER NOT TO SUPPER-IMPOSE HIT AREAS (see how we are avoiding the section's hit area)

Check the Menu section and try to use the Navigation in different ways (slowly, quick, abruptly) to see if the functionality responds. You might have to modify hit areas a couple of times.