I have to do a homework where i have to implement user registration and login in ASP.NET.(Form-Based)
I am reading my book and it says that the first thing I should do is use aspnet_regsql.exe to configure, but when I went to pick the the database I get this error:

I keep trying to fix this problem, but I dont know how. At this link I found a note that says:
The database elements that are installed in the feature database will always be owned by the SQL Server database owner account (dbo). In order to install the feature database, a SQL Server login must be permitted to the db_ddladmin and dd_securityadmin roles for the SQL Server database. However, you do not need to be a system administrator for the SQL Server in order to install the feature database.
I don't understand what I am missing.
-What should I do?
-Where can I find db_ddladmin and dd_securityadmin?
UPDATE
As the above step looks like is not able to find the database, i thought i better create it manually. so this is what i do Step 1:

Step 2:

Why is this happening?
I just need to prepare my work enviroment to use form-based authentification. I cannot do my homework if i cant configure my working enviroment first. Can someone give me a hand to fix this? Does it have to do something with permisions or other?