How to enable sandboxed solutions by using Windows PowerShell

To enable sandboxed solutions by using Windows PowerShell

1. Verify that you meet the following minimum requirements: See Add-SPShellAdmin.
2. Click Start, and then click All Programs.
3. Click Microsoft SharePoint 2010 Products.
4. Click SharePoint 2010 Management Shell.
5. At the Windows PowerShell command prompt, type the following command:

 Start-Service -Name SPUserCodeV4

6. Repeat this procedure on every server on which you want to enable sandboxed solutions

 Ref: http://technet.microsoft.com/en-us/library/ff535780(v=office.14).aspx