Error When Using Mode Provider...

i am working on asp.net, using microsoft visual web developper 2010 express. and i was trying to use the provider model. so i installed the SDK so i can use the (aspnet_regsql.exe) tool to setup the database required to use (membership, profile, role,...etc). but the first problem i met is not finding the database in the database explorer, or solution explorer. maybe because i am saving my projects in D: drive not C: drive. and when i searched in c: drive i found the database that is created by the tool aspnet_regsql.exe. therefor, i solved the problem by attaching the database manually from the solution explorer. so, is ehat i did right or wrong?? and if it is then how can i attach te database that it seems only can be created in C: drive??

after that when i tried to set up the profile provider, i edited the web.config file, created a login page and added the required code but i got this error:

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.Compiler Error Message: CS0246: The type or namespace name 'system' could not be found (are you missing a using directive or an assembly reference?)Source Error:[No relevant source lines]

i am struggling with this error for the whole last two weeks, so any help will be appreciated, thanks in advance.