'You need to use FindControl to access control on a  FormView:
Dim FirstName As TextBox = FormView1.FindControl("FirstNameTextBox")
Dim LastName As TextBox = FormView1.FindControl("LastNameTextBox")
Dim Phone As TextBox = FormView1.FindControl("PhoneTextBox")
Dim Email As TextBox = FormView1.FindControl("EmailTextBox")
SqlDataSource1.InsertParameters("FirstName").DefaultValue = FirstName
SqlDataSource1.InsertParameters("LastName").DefaultValue = LastName
SqlDataSource1.InsertParameters("Phone").DefaultValue = Phone
SqlDataSource1.InsertParameters("Email").DefaultValue = Email
SqlDataSource1.Insert()

And, this is the code for Update:

SqlDataSource1.UpdateParameters("FirstName").DefaultValue = FirstName
SqlDataSource1.UpdateParameters("LastName").DefaultValue = LastName
SqlDataSource1.UpdateParameters("Phone").DefaultValue = Phone
SqlDataSource1.UpdateParameters("Email").DefaultValue = Email
SqlDataSource1. Update()

In order to use System.DirectoryServices classes, you will need to first add a reference to System.DirectoryServices.

Imports System.DirectoryServices is required at the very top of your VB.NET code.

Function GetUserProperties(ByVal GetUser, ByVal Result)
        Dim objSearch As New DirectorySearcher
        objSearch.SearchRoot = New DirectoryEntry("LDAP://dc=domain,dc=co,dc=uk")
        objSearch.Filter = GetUser
        'cn = Full Name, givenName = First name, initials = Middle names, sn = Surname, mail = email address
        objSearch.PropertiesToLoad.AddRange(New String() {"cn", "giveName", "initials", "sn", "department", "mail", "Manager"})
        Dim objResult As SearchResult
        objResult = objSearch.FindOne()
        GetUserProperties = (objResult.Properties(Result)(0))
End Function

Replace dc=domain, dc=co, dc=uk with your AD settings.