If the Blocks Editor cannot connect to your phone, you can check a few things to solve this problem.
1: Can you open the emulator?
Click the "New emulator" button in the Blocks Editor, located right next to the "Connect to Device...." text. You can find more information about starting a new emulator here. Starting a new emulator may take some time; please allow a few minutes to let it start.
After the emulator successfully opens, find the emulator in the drop-down list. If you can find the emulator in the drop-down list, click the text "emulator" and connect to the emulator. You need to "unlock" the phone by sliding the lock on the phone's screen. If you can connect to an emulator here, move on to Step 2.
If an error message appears and says that App Inventor cannot find the "right path," it probably means you did not properly install the App Inventor Setup Software. Follow these instructions and reinstall the software again.
2. Check your phone's settings
Check your phone to make sure the USB Debugging Mode is turned on. Follow these instructions to properly set your phone's settings. If you can open an emulator but cannot connect to the phone, the USB Debugging Mode is often turned off. If you have Nexus One/S, Droid X, or other newer phone models, you will need extra steps to set your phone. Read the "Details for Various Phone Models" section in the phone setup. Also, feel free to look at the user-compiled list of compatible phones, issues, and workarounds which can be found here.
Make sure your phone's settings are correct before connecting your phone to your computer.
3. Unplug the phone and plug it back in again.
After you change the phone settings, unplug the phone and USB cable from your computer, then plug them back in again. Sometimes the problem is caused by miscommunication between the phone, USB cable, and computer. Make sure your phone's USB Debugging Mode is turned on before connecting your phone to your computer.
4. Does your phone have an SD card?
If your phone does not have a SD card, App Inventor cannot connect to the phone. Check your phone to make sure your phone has a SD card.
5. Close the Blocks Editor and reopen it
Sometimes the Blocks Editor does not update the new information properly. Close the Blocks Editor (don't worry, closing the Blocks Editor does not delete your blocks). Reopen the Blocks Editor by clicking "Open the Blocks Editor" in the upper-right hand corner of the Designer. After you click the button, a new .jnlp file will be downloaded to your computer. Open the file and start the Blocks Editor.
Make sure your phone's USB Debugging Mode is turned on before connecting your phone to your computer.
6. Check the Windows driver (for PC users only)
Check your phone's model type in the list and follow these instructions to check your Windows driver. Read the second section, Checking the driver installation, to make sure your Windows driver is working properly.
Try unplugging and replugging the phone into your computer and trying the "adb devices" command again before getting or installing device drivers, as sometimes that is sufficient to make the connection work.
7. Reinstall Java
Sometimes reinstalling Java helps solving the problem. Your computer needs to run Java 6 (also known as Java 1.6). You can download Java from http://www.java.com
8. Check your antivirus/firewall/security settings
If your computer has strong antivirus or firewall protections, some components in the App Inventor Setup Software may not work properly. Change the security settings and try reinstalling the App Inventor Setup Software here.
Still can't connect to your phone? Take a look at the troubleshooting page, or check the App Inventor User Forum for help. You may also find the solution on the Grey Button Page. Return to the Starting with App Inventor page here.