The simplest way to install a package, is to find the package in the package tree, select it, then click the "Emerge" button. This will automatically install the latest stable version of the package, along with any dependencies it has that aren't already installed.
For example, to install The GIMP, you could select "media" then "gfx" from the category list, then select the package named "gimp" and click "Emerge". Alternatively, entering "gimp" into Porthole's search bar would find all packages with "gimp" in the name, and automatically select the one named only "gimp" for you.
Occasionally you may decide that you absolutely have to have the latest version of a package and you don't care if this particular package is unstable or not. If there is a newer version of a package that is marked as unstable for your CPU architecture, you can right-click on the package and select "Add with ~arch to package.keywords" where "arch" is an abbreviation for your CPU architecture. This will enable you to install the latest unstable version of this package every time a new version appears inthe portage tree, just as you would install a package normally.
Please note that Porthole is not well tested and there is the possibility that it will chew up. mutilate, or otherwise render useless your current package.keywords file. It is recommended that you make a backup of this file (usually /etc/portage/package.keywords) before accepting keywords with Porthole. Of course, if you haven't edited it yourself, chances are it's empty to start off with anyway :).
For finer control of emerge options, use flags and versions, select the package to emerge then click the "Advanced Emerge" toolbar button. This allows you to select which version of the package you wish to install, and to modify the use flags it will be compiled with as well as selecting other options to pass to emerge, such as --fetchonly or --onlydeps. Refer to the Advanced Emerge help topic for more information.
If you just want to choose which version of a package to install yourself, and unmask only that version if need be, you can quickly and easily do this by right-clicking on the version table in the package summary (displayed in the bottom pane of the main window).
If you click on a keyword masked version of the package, it will allow you to modify package.keywords as described above. In addition however, if you click on a hard masked package, it will allow you to add it to your package.unmask file, and thus emerge it. Before you unmask a hard masked package, always read the masking reason, which is displayed as a tool-tip when you hover the mouse-cursor over the hard masked version. There is always a good reason for packages to be hard masked, and if you don't know exactly what it is, don't unmask it!