Cannot complete the install because one or more required items could not be found.
Software being installed: Force.com IDE 25.0.0.201206181021 (com.salesforce.ide.feature.feature.group 25.0.0.201206181021)
Missing requirement: Force.com IDE 25.0.0.201206181021 (com.salesforce.ide.feature.feature.group 25.0.0.201206181021) requires ‘org.eclipse.update.ui 0.0.0’ but it could not be found

対応策：
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It should be possible to fetch the deependency automatically through the marketplace but for me it’s not working…. so what would you need to do is install the org.eclipse.update.ui manually from e.g.
http://www.java2s.com/Code/Jar/o/Downloadorgeclipseupdateui32300v20100512jar.htm

上記URLからファイルダンロードして、eclipseのpluginsにコピーする