Session state can only be used when enableSessionState is set to true ... SharePoint 2010

In web.config

1. <pages enablesessionstate="true">

2. <modules runallmanagedmodulesforallrequests="true">
..
..
<remove name="Session">
<add name="Session" type="System.Web.SessionState.SessionStateModule" precondition="">

3. <pageparserpaths>
<pageparserpath virtualpath="/*" compilationmode="Always" allowserversidescript="true" includesubfolders="true"></pageparserpaths>

Save and exit.