Thank you for posting.
Elecard AVC PlugIn is intended to use with MPEG Player, DVD Player, XMuxer Pro and XMuxer Lite applications from Elecard. Please seehttp://www.elecard.com/products/products-pc/consumer/avc-plugin/
So if you need to play MP4 files in WMP you can either purchase Elecard MPEG Player (in addition to AVC PlugIn) or wait for AVC PlugIn for WMP which will be released later this month.
There is another option - Elecard AVC Streaming PlugIn for WMP allows receiving and decoding media data from network. But it doesn't support mp4 at the moment too, the version that supports that format will be available in the next release only. For more information seehttp://www.elecard.com/products/product.php?product_id=avc-streaming-plugin