the trial has no limitations in functionality. You are saying that you are calling a hosted web service, but hosted web services can't have a target so I don't know how you could assign a plugin to it. Could you send me the jitterpak so I can have a look at your configuration? You can email it to email@example.com
if you don't want to post it on the forum. If you need to run the plugin to decode some Base64 you can write the data to a global variable and then run the plugin in a success operation (or by calling out to it with RunOperation). You should also be able to assign the plugin to the Web Service Method. You can choose to call it on request and on response.
To verify that the plugin assignment has been registered on the server you can connect to the database called TranDb (use Start -> All Programs -> PostgreSQL 8.1 -> pgAdmin III) and check the contents of the tables pipelineplugininstancetab and pipelinepluginpositiontab. You should have one entry in each for each plugin that you have assigned.