If you are using firefox, then you can try this

1) Type about:config into the address bar
2) Then type general.useragent.extra.firefox into the filter bar
3) Double click it and change the value into Firefox/2.0.0.2 MEGAUPLOAD 1.0

Hope it helps. 

For mozilla firefox:

First go to http://addons.mozilla.org/firefox/59/

Click Install Now to install addon.

Restart FireFox. Tool-->User Agent Switcher--->Options--->Options

Click User Agents--->Add

Type this:

+ Description: MEGAUPLOAD

+ User Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Alexa Toolbar)

Click OK twice to finish adding User Agent.

From now, before downloading any file on megaupload just go Tools--> User Agent Switcher--->MEGAUPLOAD.

For IE:

Start/run->> type regedit -> ok

Go to: [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Internet Settings\5.0\User Agent\Post Platform]

Note: if U use IE 7

go to: [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Internet Settings\User Agent\Post Platform]

Right click on Post Platform > New > String Value > type Alexa Toolbar > ok

Now U can download from megaupload without error