Open this task link missing in approval workflow+ sharepoint 2010

1. Open up the Document List (or any list) from SharePoint Designer 2010
2. Under the "Workflow" panel (lower right), click on "New" to create a new workflow (it will be shown as empty initially until you create a new one). Give a name to your workflow if prompted.
3. You are now at the Workflow window. Click on "Edit workflow" under the "Customization" panel
4. You are now at the Editor for the workflow. At the big horizontal cursor under "Step 1", type in "Start" and enter. It will then give you a list of possible actions you could add.
5. Pick "Start Approval Process"
6. A new line/item is added for "Start Approval process on current item with these people"
7. Now click on "Approval" on the line (the underlined one)
8. You are now at the Task Process level. Now, you could customize how the task is to be performed.
9. Click on "Change the behavior of a single task"
10. Under "When a Task is Pending", there are 2 Email task notification actions (e.g. "Email task notification to Current Task:Assigned To") and these actions is used to send the approval task emails, so we need to modify the emails by modify these actions. (so click on "Current Task: Assigned To")
11. You are now in the Define E-mail Message window. This is where you define what the notifications should look like.
12. Highlight the "Open this task" text, and click on the Hyperlink button (the last button on the right on the fourth row).
13. You are now at the Hyperlink editing window. The "Display" field should says "Open this task"
14. For the "Address" field, click on the "..." button to use the String Builder tool
15. Click on "Add or Change Lookup" button
16. In the "Data Source" dropdown, pick "Current Task: <name of the workflow name>"
17. In the "Field from Source" dropdown, pick "Form_URN"  where condition ID=current list ID (OR) follow step  
18. Click ... button behind the Address option, type following string in String builder.
    http://server/site/Lists/your task list/DispForm.aspx?ID=
    Then click Add or Change Lookup on the left button of the String builder, select ID field from Current Task data source.
    So the final string should similar like this:
    http://server/site/Lists/your task list/DispForm.aspx?ID=[%Current Task:ID%]
19. Add the link to both 2 email task notification actions.
20. Click all the "OK"s and back to the Editor window. The "Open this task" should now be a hyperlink to the Form URN.
21. Go back to the Workflow Editing window (where you see the "Start Approval process on current item with these people" line), click on "these people" to define who should receive these notifications (standard procedure basically...)
22. Now you could save it and publish it, and see how it works for you!