Hi Farah,
First I would say don't use IIS or any web page or tools it provides to edit the Web.config file. It may or may not have issues with mojoPortal, I don't know because I don't use it and therefore can't help you much if you are using that tool. Web.config is just a text file it can be edited with notepad and has comments in it about a lot of the settings which you may not be seeing using this tool. See the Web.config documentation for information that may help.
Also, if you continue to have trouble, please tell more about your environment:
What operating System?
What version of IIS?
What version of MS SQL?
You do not need to create any database with a specific name, it can be named whatever you like so long as the connection string is good and it has permission to create database objects like tables, procedures etc. You do not need to set that SubSonic connection string, if you were actually reading the Web.config file instead of using this tool you would see comments telling you that you don't need to set it.
Best,
Joe