I am having same problem. Just got my Digispark Pro's today and want to use the Serial Port capabilities so I can use this via ChiliPeppr.com for doing some laser soldering. When I try to compile the DigiUSB Echo example (or anything that uses DigiUSB) I get:
Arduino: 1.5.8 (Windows

, Board: "Digispark Pro (16 Mhz)"
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:70:14: error: variable 'usbDescriptorString0' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
PROGMEM char usbDescriptorString0[] = { /* language descriptor */
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:70:14: error: conflicting types for 'usbDescriptorString0'
In file included from C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:12:0:
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.h:515:6: note: previous declaration of 'usbDescriptorString0' was here
char usbDescriptorString0[];
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:80:14: error: variable 'usbDescriptorStringVendor' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
PROGMEM int usbDescriptorStringVendor[] = {
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:80:14: error: conflicting types for 'usbDescriptorStringVendor'
In file included from C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:12:0:
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.h:521:5: note: previous declaration of 'usbDescriptorStringVendor' was here
int usbDescriptorStringVendor[];
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:89:14: error: variable 'usbDescriptorStringDevice' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
PROGMEM int usbDescriptorStringDevice[] = {
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:89:14: error: conflicting types for 'usbDescriptorStringDevice'
In file included from C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:12:0:
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.h:527:5: note: previous declaration of 'usbDescriptorStringDevice' was here
int usbDescriptorStringDevice[];
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:111:14: error: variable 'usbDescriptorDevice' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
PROGMEM char usbDescriptorDevice[] = { /* USB device descriptor */
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:111:14: error: conflicting types for 'usbDescriptorDevice'
In file included from C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:12:0:
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.h:491:6: note: previous declaration of 'usbDescriptorDevice' was here
char usbDescriptorDevice[];
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:142:14: error: variable 'usbDescriptorConfiguration' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
PROGMEM char usbDescriptorConfiguration[] = { /* USB configuration descriptor */
^
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:142:14: error: conflicting types for 'usbDescriptorConfiguration'
In file included from C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.c:12:0:
C:\Users\john\Documents\Arduino\hardware\digistump\avr\libraries\DigiUSB\usbdrv.h:497:6: note: previous declaration of 'usbDescriptorConfiguration' was here
char usbDescriptorConfiguration[];
^
Error compiling.
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