Normally not (it depends on what part of the plugins code you want to use and how the plugin initializes itself in the context in which you want to do it), the main plugin files are loaded as part of the wordpress boot process and the convention is that they will load the other files in the plugin at that time, therefor all the function declared in a plugin should be available to use after the boot was finished, and especially in a theme.
The error you are getting is probably just a syntax error in your code.