dont listen to Luis, well most of it ;) ,
wspbuilder is brill for 2003 and 2007! for 2010 make sure you have the correct version!
you need WSPBuilder Extensions 2010 BETA 1.4 which is at the bottom!
also, your using 2013 which is different to 2010 but you could give it ago! things have changed from 2010 to 2013 in how the webparts are made!
Creating a Visual WebPart in SharePoint 2013 is different than what it
was available in SharePoint 2010. In SharePoint 2010, Visual WebPart
was made of a User Control and a Webpart file which were used to
define Controls and define properties of the webpart. In SharePoint
2013 however the Visual WebPart template has Changed. Both the User
Control and WebPart classes are merged to Create one template. Also,
now you have one Visual WebPart template that can be used to Create
both Sandbox and Farm solution. In this post we will look at detailed
steps on how to create a Visual WebPart Farm solution and add the wsp
to your SharePoint site.
I do agree with Luis on the fact that you should be using vs now as its come along way from the 2003 days! but to say its old doesnt mean its no good!
have this for reference:
for 2013 webpart creating step by step:
in the example above you can test it by playing the code (debug) goto the temp folder and ull see the created wsp! copy that and use it on the 2013 server ;)