You need to use @ViewChild directive to get access to the collapse methods in your component.

@ViewChild('fixed') fixed: CollapseComponent;

Then in AfterViewInit hook (the method won't be available before view init):

Promise.resolve().then(() => {
     this.fixed.show();
 })

The promise is there to avoid Angular ExpressionChangedAfterItHasBeenCheckedError error.

2 years later, what is the easiest way to open a fixed menu programmatically ?

<div class="fixed-action-btn" style="bottom: 45px; right: 24px;">

<a class="btn-floating btn-large red waves-light"mdbWavesEffect (click)="fixed.toggle($event)">

<i class="fa fa-pencil"></i>

</a>

<div class="fixed_collapse" [mdbCollapse]="isCollapsed" #fixed="bs-collapse">

<a class="btn-floating red waves-light" (click)="fixed.toggle($event)"mdbWavesEffect><iclass="fa fa-star"></i></a>

<a class="btn-floating yellow darken-1 waves-light"mdbWavesEffect><iclass="fa fa-user"></i></a>

<a class="btn-floating green waves-light"mdbWavesEffect><iclass="fa fa-envelope"></i></a>

<a class="btn-floating blue waves-light"mdbWavesEffect><iclass="fa fa-shopping-cart"></i></a>

</div>

</div>