Hi Zeeshan,
LLBLGen ANGTE (ASP.Net GUI Templates Extended) is available as OpenSource at http://llblgenangte.codeplex.com . This is an 3rd party additional download and it's not included in the installation package. You have to download and follow the instructions.
ANGTE supports LLBLGen 3.1. Though I think it should work ok with 3.5 as well. It needs to be ported to v4.x and v5.x. I assume you want to use it in v5.1, Right? I helped to write those templates so maybe we can make an effort to update them.
If you want to download LLBLGen v3.5 you can do you if you login as Customer and go to MyAccount->Downloads->v3.5. If you don't have that option and you have a valid v5.x license I think you can request a special backward license for this. If this is your case please open a HelpDesk thread to ask about this.