If you have managed DLL, and not COM object, you shouldn't use -ComObject parameter, use just a class name from loaded assembly, or fully qualified name of class from assembly located in GAC, just like you would do in c#.
[sergeia-64]: PS C:\Users\sergeia\Documents> $o = new-object System.Text.StringBuilder
[sergeia-64]: PS C:\Users\sergeia\Documents> $o | gm
TypeName: System.Text.StringBuilder
Name MemberType Definition
---- ---------- ----------
Append Method System.Text.StringBuilder Append(char value, int repeatCount), System.Text.Stri...
AppendFormat Method System.Text.StringBuilder AppendFormat(System.IFormatProvider provider, string ...
AppendLine Method System.Text.StringBuilder AppendLine(), System.Text.StringBuilder AppendLine(st...
CopyTo Method System.Void CopyTo(int sourceIndex, char[] destination, int destinationIndex, i...
EnsureCapacity Method int EnsureCapacity(int capacity)
Equals Method bool Equals(System.Text.StringBuilder sb), bool Equals(System.Object obj)
GetHashCode Method int GetHashCode()
GetType Method type GetType()
Insert Method System.Text.StringBuilder Insert(int index, string value, int count), System.Te...
Remove Method System.Text.StringBuilder Remove(int startIndex, int length)
Replace Method System.Text.StringBuilder Replace(string oldValue, string newValue, int startIn...
ToString Method string ToString(), string ToString(int startIndex, int length)
Chars ParameterizedProperty char Chars(int index) {get;set;}
Capacity Property System.Int32 Capacity {get;set;}
Length Property System.Int32 Length {get;set;}
MaxCapacity Property System.Int32 MaxCapacity {get;}