[bugs] Cannot install extension in chrome portable version 21.0.1180.60

Please read the section "Installing plugins" within the file help.html. This file is contained in the GoogleChromePortable folder. At the beginning of this section is mentioned the following:

If you wish to have portable plugins, be aware that many plugins are linked to locally installed programs and will not function correctly in a portable environment. When testing plugins be sure to test them on a system that does not have the plugin installed locally.

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Maybe this information helps you to detect the cause better.

EDIT: Obviously my advice is not the solution, because your request refer to the installation of extensions. Sorry.

Which extension are you trying to install? And which is the error message?

The cause for your problem is the fact, that the Google Chrome browser has a built-in barrage regarding the installation of extensions, apps and user scripts. After downloading an extension appears in the browser at top a yellow marked line with following content:

Extensions, apps and user scripts can only be added from the Chrome Web Store

This is a precautionary measure by Google with the aim, that the installing of malicious extensions should be avoided. Only extensions from the Chrome Web Store can be installed without problems in the usual kind.

Now I will explain you the procedure, how you can install extensions, which you haven't downloaded from the Google Web Store. After the download of an appropriate extension you open your Windows Explorer and search after the downloaded extension. In the Google Chrome Portable browser click on the spanner on top right and choose the entry "Tools". In the appearing context menu choose the entry "extensions". Now will opened a dialog window, in which are listed all your extensions. Per drag and drop you move your downloaded extension from the Windows Explorer in your Chrome extension dialog window. In a last step you need to click on the button "Add" in a new appearing dialog box. Then the extension is installed in the Google Chrome Portable Browser.

I hope answered your question sufficiently precise.