DispForm.aspx that displays detail view of list item and EditForm.aspx that displays edit view of list item are not customizable by default. You can move to these pages but there is no Edit option in Site Actions menu. But it still possible to customize these pages, add new web parts there and so on. All you need is your web browser.
- Go to DispForm.aspx or EditForm.aspx you want to customize.
- Change the query string part of URL so that query string is like this:
??? = ID of current list item, you don’t have to change it.
- Modify pages as you want and save them.
- After first modification the Edit option in Site Actions menu should also be visible now.
I found this information from somewhere but I am not able to find the original blog entry anymore. If somebody knows where it is peale let me know so I can provide link to author here.
Update. Erwin Bastiaensen thought that maybe I found this information from his blog entry Add Web Parts to DispForm, EditForm or NewForm.aspx. I think it is possible, so credits go to Erwin. :)