First, can we clarify something? Is this a video that has been shot in Portrait format and you want to display it Landscape, or is the problem that the video is being displayed/played back "the wrong way around"?
The latter is very easy to resolve, the former not all easy without considerable compromise of the actual view as seen on screen.
For video being played the wrong way around, Effects>Orientation is what you need to use to rotate the video so it is "correct side up".
To display Portrait (9:16) video in Landscape (16:9) you need to make a number of compromises with regard to the final output. You can use the Size option of the Size/Position/Rotation effect BUT this will inevitably lead to either elements at the top and bottom of the footage being "cut off" if you drag the sides out so that they completely fill the screen and you wish to maintain the Aspect Ratio of your footage, or if you choose to not retain the original Aspect Ratio and simply drag out the sides of the images you will be left with footage that has all constituents of it appearing much, much wider than they actually are!
For additional information please read this forum thread. Although it is asking about an older version of Movie Studio the principles remain the same.
I presume that you want you project to be landscape 16:9, so that is what your project parameters should be set to on the startup screen.
To complement what Jeff indicated, the image below shows a landscape project with a portrait mode video in the preview monitor. By default, the program takes the image and makes it as large as possible without cutting off anything.
In the Source Monitor at the right, I have selected a template under Design Elements, Image border effects. You can choose one of these or make your own background. The background would have to be on track 1 and your video on track 2 or a higher numbered track.
For what Jeff indicated about using Size/Position/Rotation to get the image to fill the screen, in the image below, you can see that the width is 607.5 and the height is 1080, so nothing is cut off.
To get the image to fill the screen, change the width to 1920, enter. The height will change to be much larger, in my case 3413.3 so the top and bottom get cut off.
You can move the image up or down as you wish, just don't move it sideways. If you do, use an edge button under position to get the left edge of the image to be the left edge of the window.
Better still, watch my tutorials on using Size/Position/Rotation. Here is the first one:
Watch all parts.
You may also want to watch this one about zooming in and out of the Preview Monitor window.
In the image below, I have zoomed out the Preview Monitor so that you can see the outline of the object. The image is actually filling the entire 1920x1080 window, so you can see that the top and bottom parts have been cut off and by how much. See the above tutorial to learn how to do this.
On the few occassions I have to display portrait in a landscape project, I use either:
One of the progams presets in Templates, Design Elements, Image border effects basic, however many have animation backgrounds which I do not like.
For a static background I utilise the image/video clip twice as shown in the image below
Note this is from Movie Studio, however it also applies to Video Pro X.
Background image/video - you can use the same image/video clip as the portrait image/video or any other image/video you wish. My preference in most cases where the portrait object is a video clip, is to use an image exported from the video clip, using VPX to avoid colour clashes.
The green arrow indicated setting uncouples the height and width to stretch the background image/video clip. I prefer this to enlarging the whole image/video clip to fill the background.
Blurring - you can use any of the blur, sand, erosion etc effects as you desire.
Portrait image/video - the blue arrow indicated setting maintains the aspect ratio of the centre portrait image, by increasing the height or width and vertical position you can, in effect, crop the image as can be seen in the image above.