Deleting an "in use" folder

You could try repairing permissions in Disk Utility.

The Apple advice is here

http://docs.info.apple.com/article.html?artnum=106272

And there is also

http://www.macmaps.com/software.html#FET

but Read Carefully the Caution...

We can't see the name or location of the folder. Or what the files inside are or group it belongs to.

it isn't about the system or library, so "repair permissions" isn't likely going to do anything, though you might want to delete using root and secure delete. Might want to change ownership and group to yourself and then 'apply to enclosed files and folders."

I sometimes find Adobe will lock a file or folder, or owned by root, and use Path Finder to nuke it.

Sometimes you just have to restart if it is owned by system.

I had an older copy of Palm Desktop - thing was a mess to get rid of even though never used or opened in 4 yrs.

Folders or packages (applications and even some data files like Quicken have or are packages) can have background apps or processes.

But, without knowing what it is, just "shooting in the dark." 😟

Almost any folder is easy to remove in Terminal, using sudo.
But you must know what you're doing.

Very likely that the restart did the trick.

If you have locked files in the trash, you can hold down the Option key while emptying the trash to delete the files and you won't get the message saying that the trash couldn't be emptied because some files are locked.

For your problem with files/folder in use, you can use the lsof command in Terminal to discover which process has the files/folder open. Just type 'lsof' and a space character in the Terminal window and then drag the offending file or folder onto the terminal window. When you press Return, you'll see a bunch of info such as the name and process ID of the process that has the file/folder open. You can then use the Terminal or Activity Monitor to kill that process and shouldn't have to restart.