下面的文字摘自VS2008的readme, 官方文档

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Using Office SharePoint workflow templates on a 64-bit operating system is unsupported and causes an exception.

Visual Studio 2008 throws two exceptions if you try to create a SharePoint workflow project (SharePoint 2007 Sequential Workflow or SharePoint 2007 State Machine Workflow) on a 64-bit computer. The first exception message is: "Object reference not set to an instance of an object". The second message is: "SharePoint site location entered is not valid. The SharePoint site at http://<serverpath> could not be found. Verify that you have typed the URL correctly".

解决方案

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Do not use the Visual Studio SharePoint workflow templates on a computer that is running a 64-bit operating system.

WorkAround

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这里包含一个workaround

http://pointstoshare.spaces.live.com/blog/cns!AEC42F315B4528B0!3018.entry?wa=wsignin1.0&sa=810023154

步骤如下:

1. 安装wspbuilder

2. In Visual Studio 2008  Click, File > New > Project
3. Select WSPBuilder and then Select then WSPBuilder Project with Workflow
4. Click OK
5. In the Solution Explorer window Right Click on the Projet Name (for example, WSPBuilderProject1)
    > Add > Sequential Workflow
6. In the add New Item Window that just open, select Workflow categorie and the Sequential Workflow
7. And then , click on the Add button.

这里包含另一个workaround, 不过与上一个差不多.

http://blah.winsmarts.com/2008-7-Authoring_SharePoint_2007_Workflows_using_VS2008.aspx

Visual Studio 2008 Readme

http://download.microsoft.com/download/9/a/e/9ae0f6cc-7032-408e-9ca7-989f9e4af4ec/VS2008Readme.htm#Visual Studio Tools for Office

VS2008 SharePoint Workflows do not work on 64-bit machines

http://connect.microsoft.com/VisualStudio/feedback/details/345347/vs2008-sharepoint-workflows-do-not-work-on-64-bit-machines