Error installing plugins

I'm assuming you mean the profile plugins. If you mean the plugins from the main installation package, let us know.

As for what to check, make sure you've already unzipped the main plugin package. Specifically, if the file you're trying to use ends in ...unzip_first.zip, make sure you unzip it. Then, each plugin installs individually, and you should only install the profile plugins for the 3rd party extensions you have installed.

Beyond that, you may need to check that your temp folder is writeable. If not, that could cause Joomla to not see the XML file correctly. However, if you already installed the component, that's likely not the issue.

Hope that helps,
Alex