To enable session for ASP.NET application, you would set the attribute “enableSession” of element configuration/system.web/pages, an example is given below from a default SharePoint web application configuration file.
<pages enableSessionState="true" enableViewState="true" enableViewStateMac="true" validateRequest="false" pageParserFilterType="Microsoft.SharePoint.ApplicationRuntime.SPPageParserFilter, Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" asyncTimeout="7">
<namespaces>
<remove namespace="System.Web.UI.WebControls.WebParts" />
</namespaces>
<tagMapping>
<add tagType="System.Web.UI.WebControls.SqlDataSource, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" mappedTagType="Microsoft.SharePoint.WebControls.SPSqlDataSource, Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
</tagMapping>
</pages>
Also you need to add http module entry into element configuration/system.web/httpModules, an example is given below.
<httpModules>
<clear />
...
<add name="Session" type="System.Web.SessionState.SessionStateModule" />
</httpModules>