After running that command, there should be a directory called "forums-portlet" in your working directory. This contains the complete forums-portlet package.
To install this package with the OpenACS Package Manager (APM), its directory must be located under <service>/packages. So you either need to move the directory there, or check it out under <service>/packages. If the directory is there, it should appear in the APM under "Install from local".
Note that many packages have dependencies on other packages which are necessary for a successful install. The APM helpfully lists all necessary packages and complains about those that are missing. You would usually have to check them out from CVS as well.
Instead of reloading the APM page any number of times and checking out one package after the other, you can also look at the .info file in each package directory. Look for lines saying "required" and check out the packages given there. You can check out multiple packages with a single cvs command, just list them all: "cvs ... forums-portlet project-manager ..."
Repeat for each new package until all dependencies are fulfilled.
Another way would be to just simply check out the whole OpenACS directory, if you can live with a crowded packages/ directory.