Well, the 2.8 version of the plugin is fully compatible with WP 3.5. and that error would have nothing to do with WP compatibility or PHP. It's just saying it is calling a function that it cannot find.
The function where that function is called calls two functions, wpmem_a_meta_box and wpmem_a_rss_box. Both of those are found in admin/dialogs.php. So if that file was not loaded properly, then you would actually get an error message for the meta box function first.
So, knowing that the plugin is not broken and that there are no known bugs in this area, there are a few possibilities:
1. This function was added late in the 2.8 development and was not included in all of the beta release candidates that were released. So if you had one of those installed at any point it could be possible that you have one updated file and one out of date.
2. If this was a new install or an update via the WP plugin panel, sometimes errors occur during the download. If that was the case, it could be possible that you have a corrupted file in the installation.
3. You didn't mention this and I assume that this is not the case, but user edits to any of the files could result in errors.
In any of these (or other possibilities), my recommendation would be a clean install. Deactivate the plugin, then use the delete option in the WP plugin panel (this will run the plugin uninstall function to clear out any traces of the plugin). Then reinstall. You could reinstall via the WP plugin panel, but I would recommend downloading the zip package, just in case #2 above was the problem.