Harsen,
(sorry for the delay - I've been busy and haven't had alot of time to look at your spoiler problem)
This morning I opened your CATIA file and took a look at what you have. Here's some notes on what I observed:
a. when I tried to use the Multi-Section Solid. As other people have commented, I verified the 3 section curves directions (arrows) were all pointing in the same direction, and I also selected closing points that were along the same guide curve. I did not use the guide curves, as I wanted to see how the section curves would effect the result. I received a topology error message indicating bad geometry.
b. I switched to the GSD workbench, and used the Multi-Section Surface tool to make a surface through the 3 sketches. This time it made a surface, but it did not follow your guide curves (as expected).
c. I edited the surface and added the 10 guide curves, with good results.
d. I noticed on the one end of the spoiler, you have a multi-section solid. This solid is not tangent to my surface, but I suspect it should be?
e. I edited the surface again and added the Sketch.29 as the 4th section. Since the guide curves do not extend to this sketch, I removed all of them from the surface definition. The result of this surface is attached. This surface can be converted into a solid by using the CLOSE SURFACE icon in Part Design.
Here's my suggestions for the next time you have to model a spoiler:
1. Since the spoiler is symmetrical (like your other geometry), make half sections and then mirror the final result.
2. Make the section geometry tangent within the sketches.
3. The sections should probably also have good curvature continuity, so I'd use conic curves in the corners instead of the circular arcs. (this is a design thing - so it's optional)
4. For parts like this, I like making surfaces first and then make solids from them. Other users might disagree. I find the Multi-Section Surface tool gives me more capabilities than the solid equivalent.
5. Often, guide curves are not necessary so don't overcomplicate the geomety. I think you have different corner radii in the sections, so the guide curves probably help with your spoiler.
6. Another approach might be to make individual surfaces of the top, left and bottom walls of the left side of the spoiler. Then add fillets to corner off the sharp edges. Mirror to get the right side.
7. Model each part in a separate CATPart file. Assemble all the individual parts into a CATProduct.
These are just suggestions - as the designer, you (and your professor) have the final say in how it's done.
Keep us updated on your project - many of us (myself included) would like to see some pictures of the final result.
it's smarter to mirror the whole part, becouse it's symetric, and it's easier... I am now still working the ''nose'' abit of desinging mistakes i made, and I correct them now... I have also desingned a wheel, here is the picture... post your comments... 
