Please see this link.. it says
Check if your computer had your phone driver. I assume that you use Windows. Open Device Manager
At a command prompt, in the Start Search box, or in the Run box, type the following command: mmc devmgmt.msc
. Otherwise : check link
If you find your phone name or ADB Interface so you got driver. Try to restart DDMS
or restart your Eclipse
.
If you don't find your device or see something Other device with yellow question mark so try to install driver by browsing on your phone to Settings - Developer Options - Enable developer option - Enable USB debugging - Plug your phone to computer. Windows will ask you to install driver.
3, Update Google USB driver in Android SDK Manager (in Eclipse) or search Samsung Galaxy Ace GT-S5830i driver
4.Some weird case i got on some devices.
Nexus S: disable Developer option. Restart phone. Enable again. Windows ask to install driver or you will see it in Eclipse.
HTC phone or Samsung : change Connectivity Option on phone to Charge only (Swipe status bar. Check notifications displayed). Some phone set it to some kind of Sync data so Windows can not recognize device.
Another answer in the same link tells you the steps to select the avd manually..It is as below..
Step 1
Go to Run Configuration
Step2
Select the Target
Step 3
Choose Always prompt to pick device.Click Ok
Step 4
Now run your project you should see the emulator and your device. Select your device and click ok.
Check these steps.. Hope that it helps..