Here are the steps:
Step 1: Make sure to have installed Guest Additions. Click here for instructions.
Step 2: From the VirtualBox’s menu go to Devices>Shared Folders…
Step 3: A window will show up. In this window click the [+] button
Step 4: Select the path from your hard disc, then specify a name
Step 5: Click [OK], click [OK]
Now to actually mount these folders from Ubuntu. First create a mounpoint, that is, a directory in your Ubuntu which will reflect the shared folder from Windows
Step 6: Go to Terminal and type sudo mkdir /path
Note: path is the directory you want in Ubuntu. Now mount the shared folder.
Step 7: Type sudo mount -t vboxsf folder-name /path
Note: folder-name is the name you specified earlier in VBox
How to test:
Step 1: Go to your host OS and open the directory
Step 2: Create or paste some files here
Step 3: Go to Ubuntu
Step 4: Click on Places>Computer then search until you get to the path you created earlier in Ubuntu
I tried pasting a large AVI movie in Windows Explorer to my shared folder, went back to Ubuntu, reloaded the shared folder there, and presto, the AVI movie was there. I double-clicked it and out came the Movie Player: Totem ready to play the AVI movie.