IIRC I had this same problem and I just created my own plugin by creating the signup form on the mailchimp website:
Create Form > Signup Form > create embed code for small form
Then create the form elements you need and copy that embed code into your plugin .php file. My test of this created the following html:
<div class="mc-field-group" style="margin: 1.3em 5%;clear: both;overflow: hidden;">
<label class="input-group-label" style="display: block;margin: .3em 0;line-height: 1em;font-weight: bold;">Interested in: </label>
<div class="input-group" style="padding: .7em .7em .7em 0;font-size: .9em;margin: 0 0 1em 0;">
<ul style="margin: 0;padding: 0;">
<li style="list-style: none;overflow: hidden;padding: .2em 0;clear: left;display: block;margin: 0;"><input type="checkbox" value="1" name="group[1][1]" id="mce-group[1]-1-0" style="margin-right: 2%;padding: .2em .3em;width: auto;float: left;z-index: 999;"><label for="mce-group[1]-1-0" style="display: block;margin: .4em 0 0 0;line-height: 1em;font-weight: bold;width: auto;float: left;text-align: left !important;">banana</label></li>
<li style="list-style: none;overflow: hidden;padding: .2em 0;clear: left;display: block;margin: 0;"><input type="checkbox" value="2" name="group[1][2]" id="mce-group[1]-1-1" style="margin-right: 2%;padding: .2em .3em;width: auto;float: left;z-index: 999;"><label for="mce-group[1]-1-1" style="display: block;margin: .4em 0 0 0;line-height: 1em;font-weight: bold;width: auto;float: left;text-align: left !important;">apple</label></li>
</ul>
</div>
</div>
<div id="mce-responses" style="float: left;top: -1.4em;padding: 0em .5em 0em .5em;overflow: hidden;width: 90%;margin: 0 5%;clear: both;">
<div class="response" id="mce-error-response" style="display: none;margin: 1em 0;padding: 1em .5em .5em 0;font-weight: bold;float: left;top: -1.5em;z-index: 1;width: 80%;background: FBE3E4;color: #D12F19;"></div>
<div class="response" id="mce-success-response" style="display: none;margin: 1em 0;padding: 1em .5em .5em 0;font-weight: bold;float: left;top: -1.5em;z-index: 1;width: 80%;background: #E3FBE4;color: #529214;"></div>
</div>
<div><input type="submit" value="Subscribe" name="subscribe" id="mc-embedded-subscribe" class="btn" style="clear: both;width: auto;display: block;margin: 1em 0 1em 5%;"></div>