Having used up the free period, I have now successfully subscribed to a 4€ monthly pass using the WAP portal.
1 Select 'My Account'
2 Scroll down and look for 'Monthly Pass' - 'Miniaio paso' (apologies to any Greek reading this.) Select that.
3 You get a page of details on charges. Look again for 'Monthly Pass' and select it.
4 You get a page telling you what you'll get and that it'll cost 4€. Select 'Activate' - 'Energopoiesi' (like 'energize'). The other button 'Akuro' is cancel.
5 You should get a 'successful activation' - 'epituxis ...' page and your credit should have been reduced by 4€. There is no confirming SMS.
I don't think that this renews automatically, or that I'll get a warning SMS when it expires, but I won't know for a month.
I know that some sailors like me read this site, so I've collected together the Cosmokarte and My View'n'Web stuff, and added some information about using it for sailing weather forecasts, here:
Greek Internet Weather Yacht Vigdis
(Corrections and comments welcome.)
Dave