Did you go through this Installing the ADT Plugin?
Start Eclipse, then select Help >
Install New Software.
In the Available Software dialog,
click Add....1.
In the Add Site dialog that appears,
enter a name for the remote site
(for example, "Android Plugin") in
the "Name" field.2.
In the "Location" field, enter this
URL:
https://dl-ssl.google.com/android/eclipse/
Note: If you have trouble acquiring
the plugin, you can try using "http"
in the URL, instead of "https" (https
is preferred for security reasons).4.
Click OK.
Back in the Available Software view,
you should now see "Developer Tools"
added to the list.6.
Select the checkbox next to
Developer Tools, which will
automatically select the nested
tools Android DDMS and Android
Development Tools.7.
Click Next.8.
In the resulting Install Details
dialog, the Android DDMS and
Android Development Tools features
are listed.9.
Click Next to read and accept the
license agreement and install any
dependencies, then click Finish.
Restart Eclipse.10.
This problem is caused because you have some plugins missing that are required by android tools. You can fix this by either installing a version of Eclipse which comes bundled with a lot of the eclipse plugins, or by tracking down the plugins yourself.
To do this, run eclipse, and go to Help->Install New software. Attempt to install the Android SDk, and you will be informed of what is missing, e.g:
*Missing requirement: Android Development Tools 0.9.6.v201002051504-24846 (com.android.ide.eclipse.adt.feature.group 0.9.6.v201002051504-24846) requires *'org.eclipse.gef 0.0.0' but it could not be found
So we need to install org.eclipse.gef . A quick Google for org.eclipse.gef, gives us www.eclipse.org/gef
Follow the link, and on the left hand side you will see a link for "Update Manger", follow that link...
You then have a list of URI's similar to the one for the Android plugin. Copy the one ending in "releases"( http://download.eclipse.org/tools/gef/updates/releases/ if you can't be bothered ), and go back to eclipse. Paste this URI into the "Work with" box, as you did with the Android plugin, and install the components that come up.
Continue to do this until all of the Android dependencies are met, and the Android plugin will install. You will need to locate the correct locations for each of the required dependencies.
Head over to Help -> Install New Software. Click on Available software sites. Delete the Android repo. Uncheck Indigo & Eclipse updates & recheck them. Now head back to Help -> Check for updates. Once done, add the Android repo again. Accept the license & you should be good to go.
(Had to do the same yesterday after getting Indigo)
Best Answer
you MUST run eclipse as administrator to install plugins under vista or 7!!! right click on the eclipse.exe file (not the shortcut). if you still have problems, try the step below.
one reason this can happen is that you first need to include the release compatible with your version of eclipse:
http://download.eclipse.org/releases/juno (or helios, ganimede, indigo, etc)
in case you are missing the android sdk, download from http://developer.android.com/sdk/index.html
after installing it, you must go inside the folder c:/Program Files (x86)/Android/android-sdk/platforms/ and create a new (empty) folder 'android-7'. then create a folder named temp in c:/Program Files (x86)/Android/android-sdk/ and you will be prompted to download SDK when you start eclipse again (always as administrator)
finally, restart and you should be ok
PS: you must update your eclipse (under HELP) and then exit it. restart in admin mode and open your Android SDK packages (not HELP, the tab on its left [not at my workstation right now, will reedit later]).
without installing basically everything there (some 3 to 12 hours, depending on your connection speed), you will not have a complete environment and some compilation simply will not work.