Starting from your character in 3dsMax, select the character skeleton and export in FBX format (character in t-pose, default export settings of FBX plugin, Y-up) IMPORTANT! If you are using Trinigy demo character you need to export in FBX only and all the joints that are actually animated in the character, thus for example removing all IK controls. This is also a general rule: Mixamo only needs the FK joints of your rig. Make sure your character is in t-pose on frame 0 of the scene.
Select a Motion Model - Mixamo has hundreds of online motions - Many more being added each week
Design & Synthesize Motion using built in sliders and controls
Upload your FBX model - Most likely your CAT character will be automatically mapped to our standard skeleton
Final Preview & Download - Preview to your heart's content - Only download when you're satisfied, in FBX format
Import the downloaded FBX file back into 3dsMax File > Import and Select "Update Scene" in the FBX import plugin. Select take mixamo.com and check the "Fill timeline" to update the timeline of your scene. Export the scene in .MODEL format.
Import the animation on a playable character Select Shape tab on the left, Select Entity and your Character is highlighted. On attached components window on the right top corner select Playable character. Select the properties tab on the bottom right corner and on the "Model File" choose the .MODEL file just exported in 3dsMax on step 6. On the components tab you can assign the desired motions to your character controls.