Workflows are not available after we upgrade from 2007 to 2010 using database attach method

This is a very critical issue that most of the users have faced after the migration from Sharepoint 2007 to 2010

After upgrading the SharePoint 2007 Database via database attach method to SharePoint 2010, You might find that the 2007 Approval workflows are no longer available for users to create new workflows.

In SharePoint 2010 the 2007 workflows are there to allow any running workflows to complete, but by default creating new instances of the workflows is disabled as the expectations are that you will want to move forward to the SharePoint 2010 workflows.

Resolution 

1 .  The following steps must be taken to enable 2007 workflows after upgrading to SharePoint 2010:

 =>  Go to Site Actions ->Site Settings->Site Collection Administration->Site collection features

  Activate SharePoint 2007 Workflows  

=>  Remove ‘none’ from <AssociationCategories>none</AssociationCategories>

 =>  14\template\features\ReviewWorkflows\Reviewapproval.xml

 => 14\template\features\ReviewWorkflows\ReviewFeedback.xml

 =>  14\template\features\SignatureWorkflow\Signatures.xml

 =>  Associate Workflow

  Go to Shared Documents, Library Settings, Add a Workflow

 =>  Select legacy Workflow.

  

Note : Please take a back up of Reviewapproval.xml,ReviewFeedback.xml,Signatures.xml before doing the modification, the above solution is not verfied after we install any new update. chances are there that these files might get replaced in future update. please have a backup of working files as well.

Reference -

http://blogs.technet.com/b/vinitt/archive/2011/02/23/moss-2007-workflows-no-longer-available-visible-when-we-upgrade-from-2007-to-2010-database-attach-method.aspx