Hi all,
    I have seen that many people are facing problems in building DDWRT firmware for WNR3500Lv2. I have been able to build it. Here I am sharing the steps I followed to perform the same.

[NOTE:- This method requires to have both 32-bit and 64-bit machine.]

    Step 1: Download toolchain to compile ddwrt.

    Step 2: Perform the following modification in "toolchain-mipsel_gcc4.1.2/mipsel-linux-uclibc/sys-include/regex.h" file in toolchain.

Original Code :-
-----------------

#if !defined _POSIX_C_SOURCE && !defined _POSIX_SOURCE && defined VMS
/* VMS doesn't have `size_t' in , even though POSIX says it
   should be there.  */
# include <stddef.h>
#endif


Modified Code :-
----------------


#if !defined _POSIX_C_SOURCE && !defined _POSIX_SOURCE && defined VMS
/* VMS doesn't have `size_t' in , even though POSIX says it
   should be there.  */
# include <stddef.h>
#endif

# include <stddef.h>

    Step 3: Download "DD-WRT Source Code (Used in WNR3500Lv2 Firmware Build)" from this link and untar it using following command,
        $ tar -zxvf dd-wrt.tgz

    Step 4: Download the "DD-WRT for WNR3500Lv2 Initial Release 1.0" from this link.

    Step 5: Untar "DDWRT_WNR3500Lv2_Final_Release_09-12-2011.tgz" using following command,

            $ unzip DDWRT_WNR3500Lv2_Final_Release_09-12-2011.tgz.zip
            $ tar -zxvf DDWRT_WNR3500Lv2_Final_Release_09-12-2011.tgz
        Go to DDWRT_WNR3500Lv2_Final_Release_09-12-2011 using following command,
            $ cd DDWRT_WNR3500Lv2_Final_Release_09-12-2011
        Make DD-WRT source code ready for Netgear WNR3500Lv2 using the following command,
            $ sh ddwrt_build_wnr3500lv2.sh
        
    Step 6: Perform the following modification the "dd-wrt/src/router/rules/pppoe.mk" file

    Original Code:-
    ---------------
        pppoe pppoe-%:
            @true
    Modifed Code:-
    ---------------
        pppoe-%:
            @true


    Step 7: Perform the follwoing modifiaction the "dd-wrt/src/router/busybox/Makefile" file

Line 421:-

    Original Code:-
    ----------------------
    config %config: scripts_basic outputmakefile FORCE          
       $(Q)mkdir -p include


    Modifed Code:-
    ---------------------
    %config: scripts_basic outputmakefile FORCE  
    $(Q)mkdir -p include

Line 1273:-

    Original Code:-
    ---------------------
    / %/: prepare scripts FORCE        
    $(Q)$(MAKE) KBUILD_MODULES=$(if $(CONFIG_MODULES),1) \

    Modified Code:-
    ----------------------
    %/: prepare scripts FORCE        
    $(Q)$(MAKE) KBUILD_MODULES=$(if $(CONFIG_MODULES),1) \


    Step 8: Export toolchain path.
    Step 9: Go to "dd-wrt/src/router".
    Step 10: Use the following command to compile the source code,
        "make -f Makefile.wnr3500lv2 configure all install"
[Note: This step should be performed in a 64-bit machine. I have used Fedora Core 15 for this purpose.]

    Step 11: After successful compilation download following utility and use these series of commands,
[Note: These steps should be performed in a 32-bit machine. I have used Fedora Core 14 for this purpose.]
        
        $ tar -zxvf DD-WRT_Image_Creation.tgz
    
        $ cd DD-WRT_Image_Creation
    
        $ sh ddwrt_image.sh

    If everything goes well you should find the firmware ("ddwrt-wnr3500lv2.chk") in same ("DD-WRT_Image_Creation") directory.