The WordPress hosted app previously available on Sourceforge has been turned off. Fortunately you can still extract the data and install wordpress in your project area. I have done this for both my profile project and for MAF. There is a helpful blog on how to do it at http://sourceforge.net/p/forge/community-docs/Migrating%20WP%20from%20Hosted%20Apps/, though it only explains how to migrate project related blogs, not user related blogs. If you want to do the latter and are unable to puzzle this out, send me a message and I will do my best to help.
I only signed up for SourceForge so that I could make MAF available as an open source program. Using the SourceForge web site is a very frustrating experience. There are so many things you can do that actually doing any of them is far more difficult than it should be. The help pages are hopeless. For example the information on how to create a new project is hopelessly incomplete, and I had to contact online support to find out how to get my source code into Subversion. I wasted several hours trying to find out how to do this, and can’t believe there is no simple step by step guide for doing this.
The trick is you have to check out the project before you can add files to it.