The type or namespace name 'Workflow' does not exist in the namespace 'Microsoft.Office' (are you missing an assembly reference

Hi Folks,

I created SharePoint reusable deigner work flow  which has just 2 state approval process.

I generated the wsp file with save template option and I successfully imported it into visual studio 2010 I just removed List from the tag and put my custom content type id

<Association Categories>List<Association Categories>

when I build the solution I got erro which mention in the title.

Thanks in advance.....