In order to flash your router you need to do the following steps: 

Download Putty client or you can use any telnet clients(try to stay away from Windows built in telnet) 
Change your local IP adress to a static 192.168.0.2 
Prepare putty telnet session -- 192.168.0.1 port 9000 
Start pinging your router throught a command line. Type ping 192.168.0.1 -n 900 (You don't really need to ping that many times) 
Power DIR-400 router on and wait for 2nd successful ping and then initiate putty telnet session. 
Press CTRL-C as soon as you see that boot script is executing(If you are using Putty ver. 0.60 or newer then you don't have to worry about this.) 
Start TFTP server with linux.bin in tftp root directory 
If you did everything right then you should see the following on the window of putty: 

== Executing boot script in 1.470 seconds - enter ^C to abort 
^C 
RedBoot>
Type ip_address RedBoot> -h 192.168.0.2 and press Enter 

You should see the following: 

IP: 192.168.0.1/255.255.255.0, Gateway: 0.0.0.0 
Default server: 192.168.0.2
Type fis init and then 

About to initialize [format] FLASH image system - continue (y/n)?
type y and press Enter. Wait until the process is complete: 

*** Initialize FLASH Image System 
And a descriptor for the configuration data size = 10000 
... Erase from 0xbffe0000-0xbfff0000: . 
... Program from 0x80ff0000-0x81000000 at 0xbffe0000: .
Type load -r -b 0x80041000 linux.bin and press Enter 

Using default protocol (TFTP) 
Raw file loaded 0x80041000-0x803cefff, assumed entry at 0x80041000
Type fis create linux and press Enter (This one will take a while. DO NOT UNPLUG YOUR ROUTER) 

... Erase from 0xbfc40000-0xbffce000: ......................................................... 
... Program from 0x80041000-0x803cf000 at 0xbfc40000: ......................................................... 
prog_ok 
flash_addr = 0xbfc40000 
mem_addr = 0x80041000 
entry_addr = 0x80041000 
length = 0x38e000 
img_size = 0x38e000 
... Erase from 0xbffe0000-0xbfff0000: . 
... Program from 0x80ff0000-0x81000000 at 0xbffe0000: .
Type fconfig and press Enter 

Run script at boot: true 
Press Enter 

Boot script: 
.. fis load -l vmlinux.bin.l7 
.. go 
Enter script, terminate with empty line
>> <- type fis load -l linux and press Enter 
>> <- type exec and press Enter 
>> <- just press Enter 
Boot script timeout (1000ms resolution): 2 
press Enter 

Use BOOTP for network configuration: false
press Enter 

Gateway IP address: 
press Enter 

Local IP address: 192.168.0.1 
press Enter 

Local IP address mask: 
press Enter 

Default server IP address: 192.168.0.100 
press Enter 

Console baud rate: 9600 
press Enter 

GDB connection port: 9000 
press Enter 

Force console for special debug messages: false  
press Enter 

Network debug at boot time: false 
press Enter 

Update RedBoot non-volatile configuration - continue (y/n)? 
type y and press Enter 

... Erase from 0xbffe0000-0xbfff0000: . 
... Program from 0x80ff0000-0x81000000 at 0xbffe0000: .
Type reset and press Enter 

Your router will be accessible at http://192.168.1.1