1.) Open Main Presentation
Go to file>>open and select the presentation that will have the bulk of the slides for the combined presentation.

2.) Click on the Slide Sorter icon in the bottom left corner of the screen, it looks like this



Position your cursor where you would like the new slides to be inserted into the
existing presentation.

3.) Insert Slides
Go to the INSERT >> Slides from files (this menu is in the extra pull down menu, just wait and you'll see it)
Browse to the presentation that has the slides you would like to insert and click okay.

You can select individual slides by holding down the ctrl key and clicking on the slides you would like to add. When you have selected all the slides you would like to inset click the INSERT button. This will only Insert the slides you had previously selected by holding down the ctrl key. If you would like to INSERT ALL of the slides from the second presentation into the first, click on the INSERT ALL button. **NOTE** -You can add slides from more than one presentation into the original just by repeating this process. The slides that are added remain in their original file also. Click Close on the file finder window.

4.) Rearrange Slides

While in the Slide Sorter view (which is where you should still be) you can easily rearrange the order of slides. Click and hold the left mouse button on a slide you would like to move and drag it to the new location you would like it to be and release the left mouse button. IF you hold down the ctrl button and click on several slides then hold the left mouse button on any one of them you can move all of those slides to a new location.