As Chaos says, ShellExecute is part of the SDK.
From the project window, go to Edit > Add Files to Project >> Library (*.Lib) and browse to ...\CVI\SDK\lib\Shell32.lib.
Add these two includes to the top of your list in the .c file.
Note: delete the space after < (I added the space to fool the forum's HTML error detection).
#include < windows.h>
#include < shellapi.h>
//....
// where ever you want to mail:
ShellExecute( NULL, NULL, "mailto:someuser@somewhere.com", NULL, NULL, 0);